resource/cloudformation_stack: Use flattenStringSet helper

[mattburgess]

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Relates #6867

Release note for CHANGELOG:

NONE

Output from acceptance testing:

$ make testacc TEST=./aws TESTARGS='-run=TestAccAWSCloudFormationStack'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -count 1 -parallel 3 -run=TestAccAWSCloudFormationStack -timeout 120m
=== RUN   TestAccAWSCloudFormationStack_dataSource_basic
=== PAUSE TestAccAWSCloudFormationStack_dataSource_basic
=== RUN   TestAccAWSCloudFormationStack_dataSource_yaml
=== PAUSE TestAccAWSCloudFormationStack_dataSource_yaml
=== RUN   TestAccAWSCloudFormationStackSetInstance_basic
=== PAUSE TestAccAWSCloudFormationStackSetInstance_basic
=== RUN   TestAccAWSCloudFormationStackSetInstance_disappears
=== PAUSE TestAccAWSCloudFormationStackSetInstance_disappears
=== RUN   TestAccAWSCloudFormationStackSetInstance_disappears_StackSet
=== PAUSE TestAccAWSCloudFormationStackSetInstance_disappears_StackSet
=== RUN   TestAccAWSCloudFormationStackSetInstance_ParameterOverrides
=== PAUSE TestAccAWSCloudFormationStackSetInstance_ParameterOverrides
=== RUN   TestAccAWSCloudFormationStackSetInstance_RetainStack
=== PAUSE TestAccAWSCloudFormationStackSetInstance_RetainStack
=== RUN   TestAccAWSCloudFormationStackSet_basic
=== PAUSE TestAccAWSCloudFormationStackSet_basic
=== RUN   TestAccAWSCloudFormationStackSet_disappears
=== PAUSE TestAccAWSCloudFormationStackSet_disappears
=== RUN   TestAccAWSCloudFormationStackSet_AdministrationRoleArn
=== PAUSE TestAccAWSCloudFormationStackSet_AdministrationRoleArn
=== RUN   TestAccAWSCloudFormationStackSet_Description
=== PAUSE TestAccAWSCloudFormationStackSet_Description
=== RUN   TestAccAWSCloudFormationStackSet_ExecutionRoleName
=== PAUSE TestAccAWSCloudFormationStackSet_ExecutionRoleName
=== RUN   TestAccAWSCloudFormationStackSet_Name
=== PAUSE TestAccAWSCloudFormationStackSet_Name
=== RUN   TestAccAWSCloudFormationStackSet_Parameters
=== PAUSE TestAccAWSCloudFormationStackSet_Parameters
=== RUN   TestAccAWSCloudFormationStackSet_Parameters_Default
    TestAccAWSCloudFormationStackSet_Parameters_Default: provider_test.go:55: this resource does not currently ignore unconfigured CloudFormation template parameters with the Default property
--- SKIP: TestAccAWSCloudFormationStackSet_Parameters_Default (0.00s)
=== RUN   TestAccAWSCloudFormationStackSet_Parameters_NoEcho
    TestAccAWSCloudFormationStackSet_Parameters_NoEcho: provider_test.go:55: this resource does not currently ignore CloudFormation template parameters with the NoEcho property
--- SKIP: TestAccAWSCloudFormationStackSet_Parameters_NoEcho (0.00s)
=== RUN   TestAccAWSCloudFormationStackSet_Tags
=== PAUSE TestAccAWSCloudFormationStackSet_Tags
=== RUN   TestAccAWSCloudFormationStackSet_TemplateBody
=== PAUSE TestAccAWSCloudFormationStackSet_TemplateBody
=== RUN   TestAccAWSCloudFormationStackSet_TemplateUrl
=== PAUSE TestAccAWSCloudFormationStackSet_TemplateUrl
=== RUN   TestAccAWSCloudFormationStack_basic
=== PAUSE TestAccAWSCloudFormationStack_basic
=== RUN   TestAccAWSCloudFormationStack_disappears
=== PAUSE TestAccAWSCloudFormationStack_disappears
=== RUN   TestAccAWSCloudFormationStack_yaml
=== PAUSE TestAccAWSCloudFormationStack_yaml
=== RUN   TestAccAWSCloudFormationStack_defaultParams
=== PAUSE TestAccAWSCloudFormationStack_defaultParams
=== RUN   TestAccAWSCloudFormationStack_allAttributes
=== PAUSE TestAccAWSCloudFormationStack_allAttributes
=== RUN   TestAccAWSCloudFormationStack_withParams
=== PAUSE TestAccAWSCloudFormationStack_withParams
=== RUN   TestAccAWSCloudFormationStack_withUrl_withParams
=== PAUSE TestAccAWSCloudFormationStack_withUrl_withParams
=== RUN   TestAccAWSCloudFormationStack_withUrl_withParams_withYaml
=== PAUSE TestAccAWSCloudFormationStack_withUrl_withParams_withYaml
=== RUN   TestAccAWSCloudFormationStack_withUrl_withParams_noUpdate
=== PAUSE TestAccAWSCloudFormationStack_withUrl_withParams_noUpdate
=== RUN   TestAccAWSCloudFormationStack_withTransform
=== PAUSE TestAccAWSCloudFormationStack_withTransform
=== CONT  TestAccAWSCloudFormationStack_dataSource_basic
=== CONT  TestAccAWSCloudFormationStackSet_Tags
=== CONT  TestAccAWSCloudFormationStack_allAttributes
    TestAccAWSCloudFormationStack_dataSource_basic: data_source_aws_cloudformation_stack_test.go:16: Step 1/1 error: Error running pre-apply plan: 
        Error: Failed describing CloudFormation stack (tf-acc-ds-basic-215160101747935250): ValidationError: Stack with id tf-acc-ds-basic-215160101747935250 does not exist
        	status code: 400, request id: 85383c67-c641-40d1-a26b-625c26e4478d
        
        
--- FAIL: TestAccAWSCloudFormationStack_dataSource_basic (4.16s)
=== CONT  TestAccAWSCloudFormationStack_withTransform
--- PASS: TestAccAWSCloudFormationStack_withTransform (35.76s)
=== CONT  TestAccAWSCloudFormationStackSet_basic
--- PASS: TestAccAWSCloudFormationStackSet_basic (16.17s)
=== CONT  TestAccAWSCloudFormationStackSet_Parameters
--- PASS: TestAccAWSCloudFormationStackSet_Tags (69.30s)
=== CONT  TestAccAWSCloudFormationStackSet_Name
--- PASS: TestAccAWSCloudFormationStack_allAttributes (76.67s)
=== CONT  TestAccAWSCloudFormationStackSet_ExecutionRoleName
--- PASS: TestAccAWSCloudFormationStackSet_Name (30.97s)
=== CONT  TestAccAWSCloudFormationStack_withUrl_withParams_noUpdate
--- PASS: TestAccAWSCloudFormationStackSet_ExecutionRoleName (32.09s)
=== CONT  TestAccAWSCloudFormationStackSet_Description
--- PASS: TestAccAWSCloudFormationStackSet_Parameters (68.37s)
=== CONT  TestAccAWSCloudFormationStack_withUrl_withParams_withYaml
--- PASS: TestAccAWSCloudFormationStackSet_Description (34.39s)
=== CONT  TestAccAWSCloudFormationStack_withUrl_withParams
--- PASS: TestAccAWSCloudFormationStack_withUrl_withParams_noUpdate (83.46s)
=== CONT  TestAccAWSCloudFormationStack_withParams
--- PASS: TestAccAWSCloudFormationStack_withUrl_withParams_withYaml (70.05s)
=== CONT  TestAccAWSCloudFormationStack_defaultParams
--- PASS: TestAccAWSCloudFormationStack_defaultParams (62.14s)
=== CONT  TestAccAWSCloudFormationStackSet_AdministrationRoleArn
--- PASS: TestAccAWSCloudFormationStack_withUrl_withParams (129.51s)
=== CONT  TestAccAWSCloudFormationStackSet_disappears
--- PASS: TestAccAWSCloudFormationStackSet_disappears (13.86s)
=== CONT  TestAccAWSCloudFormationStackSetInstance_disappears_StackSet
--- PASS: TestAccAWSCloudFormationStackSet_AdministrationRoleArn (32.75s)
=== CONT  TestAccAWSCloudFormationStackSetInstance_RetainStack
--- PASS: TestAccAWSCloudFormationStack_withParams (117.23s)
=== CONT  TestAccAWSCloudFormationStack_yaml
--- PASS: TestAccAWSCloudFormationStack_yaml (62.30s)
=== CONT  TestAccAWSCloudFormationStackSetInstance_ParameterOverrides
--- PASS: TestAccAWSCloudFormationStackSetInstance_disappears_StackSet (120.11s)
=== CONT  TestAccAWSCloudFormationStackSetInstance_basic

--- PASS: TestAccAWSCloudFormationStackSetInstance_RetainStack (158.44s)
=== CONT  TestAccAWSCloudFormationStackSet_TemplateUrl
--- PASS: TestAccAWSCloudFormationStackSet_TemplateUrl (41.77s)
=== CONT  TestAccAWSCloudFormationStackSetInstance_disappears
--- PASS: TestAccAWSCloudFormationStackSetInstance_basic (118.82s)
=== CONT  TestAccAWSCloudFormationStack_basic
--- PASS: TestAccAWSCloudFormationStack_basic (63.82s)
=== CONT  TestAccAWSCloudFormationStackSet_TemplateBody
--- PASS: TestAccAWSCloudFormationStackSetInstance_ParameterOverrides (230.84s)
=== CONT  TestAccAWSCloudFormationStack_dataSource_yaml
    TestAccAWSCloudFormationStack_dataSource_yaml: data_source_aws_cloudformation_stack_test.go:101: Step 1/1 error: Error running pre-apply plan: 
        Error: Failed describing CloudFormation stack (tf-acc-ds-yaml-8018489908161910833): ValidationError: Stack with id tf-acc-ds-yaml-8018489908161910833 does not exist
        	status code: 400, request id: d3d8103b-22b2-431c-ada8-3dd226c7bc88
        
        
--- FAIL: TestAccAWSCloudFormationStack_dataSource_yaml (2.74s)
=== CONT  TestAccAWSCloudFormationStack_disappears
--- PASS: TestAccAWSCloudFormationStackSetInstance_disappears (123.99s)
--- PASS: TestAccAWSCloudFormationStackSet_TemplateBody (34.58s)
--- PASS: TestAccAWSCloudFormationStack_disappears (64.17s)
FAIL
FAIL	github.com/terraform-providers/terraform-provider-aws/aws	661.037s
FAIL
make: *** [GNUmakefile:28: testacc] Error 1

The datasource-related failures look to be latent issues; they fail in the same way on current master.

[anGie44]

LGTM @mattburgess 🚀

[ghost]

This has been released in version 3.20.0 of the Terraform AWS provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template for triage. Thanks!

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. Thanks!