Support resetting of parameter group values

[ryndaniels]

This also fixes a bug in TestAccAWSDBParameterGroup_basic where the "update" didn't actually get applied because a new description in the second test config forced a new resource instead.

(This pulls in commits from #5728)

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Closes #661

Release note for CHANGELOG:

BUG FIXES:
* resource/aws_db_parameter_group - Support resetting parameter group values
* resource/aws_rds_cluster_parameter_group - Support resetting parameter group values

Output from acceptance testing:

$ make testacc TESTARGS="-run=TestAccAWSDBParameterGroup_"            
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./... -v -count 1 -parallel 20 -run=TestAccAWSDBParameterGroup_ -timeout 120m
?       github.com/terraform-providers/terraform-provider-aws   [no test files]
=== RUN   TestAccAWSDBParameterGroup_basic
=== PAUSE TestAccAWSDBParameterGroup_basic
=== RUN   TestAccAWSDBParameterGroup_limit
=== PAUSE TestAccAWSDBParameterGroup_limit
=== RUN   TestAccAWSDBParameterGroup_Disappears
=== PAUSE TestAccAWSDBParameterGroup_Disappears
=== RUN   TestAccAWSDBParameterGroup_namePrefix
=== PAUSE TestAccAWSDBParameterGroup_namePrefix
=== RUN   TestAccAWSDBParameterGroup_generatedName
=== PAUSE TestAccAWSDBParameterGroup_generatedName
=== RUN   TestAccAWSDBParameterGroup_withApplyMethod
=== PAUSE TestAccAWSDBParameterGroup_withApplyMethod
=== RUN   TestAccAWSDBParameterGroup_Only
=== PAUSE TestAccAWSDBParameterGroup_Only
=== RUN   TestAccAWSDBParameterGroup_MatchDefault
=== PAUSE TestAccAWSDBParameterGroup_MatchDefault
=== CONT  TestAccAWSDBParameterGroup_basic
=== CONT  TestAccAWSDBParameterGroup_withApplyMethod
=== CONT  TestAccAWSDBParameterGroup_generatedName
=== CONT  TestAccAWSDBParameterGroup_MatchDefault
=== CONT  TestAccAWSDBParameterGroup_namePrefix
=== CONT  TestAccAWSDBParameterGroup_Only
=== CONT  TestAccAWSDBParameterGroup_Disappears
=== CONT  TestAccAWSDBParameterGroup_limit
--- PASS: TestAccAWSDBParameterGroup_Disappears (28.98s)
--- PASS: TestAccAWSDBParameterGroup_Only (37.76s)
--- PASS: TestAccAWSDBParameterGroup_generatedName (41.40s)
--- PASS: TestAccAWSDBParameterGroup_namePrefix (41.46s)
--- PASS: TestAccAWSDBParameterGroup_MatchDefault (44.22s)
--- PASS: TestAccAWSDBParameterGroup_withApplyMethod (46.60s)
--- PASS: TestAccAWSDBParameterGroup_basic (111.70s)
--- PASS: TestAccAWSDBParameterGroup_limit (122.63s)
PASS
ok      github.com/terraform-providers/terraform-provider-aws/aws       124.963s


make testacc TESTARGS="-run=TestAccAWSDBClusterParameterGroup_"         
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./... -v -count 1 -parallel 20 -run=TestAccAWSDBClusterParameterGroup_ -timeout 120m
?       github.com/terraform-providers/terraform-provider-aws   [no test files]
=== RUN   TestAccAWSDBClusterParameterGroup_basic
=== PAUSE TestAccAWSDBClusterParameterGroup_basic
=== RUN   TestAccAWSDBClusterParameterGroup_withApplyMethod
=== PAUSE TestAccAWSDBClusterParameterGroup_withApplyMethod
=== RUN   TestAccAWSDBClusterParameterGroup_namePrefix
=== PAUSE TestAccAWSDBClusterParameterGroup_namePrefix
=== RUN   TestAccAWSDBClusterParameterGroup_namePrefix_Parameter
=== PAUSE TestAccAWSDBClusterParameterGroup_namePrefix_Parameter
=== RUN   TestAccAWSDBClusterParameterGroup_generatedName
=== PAUSE TestAccAWSDBClusterParameterGroup_generatedName
=== RUN   TestAccAWSDBClusterParameterGroup_generatedName_Parameter
=== PAUSE TestAccAWSDBClusterParameterGroup_generatedName_Parameter
=== RUN   TestAccAWSDBClusterParameterGroup_disappears
=== PAUSE TestAccAWSDBClusterParameterGroup_disappears
=== RUN   TestAccAWSDBClusterParameterGroup_only
=== PAUSE TestAccAWSDBClusterParameterGroup_only
=== CONT  TestAccAWSDBClusterParameterGroup_basic
=== CONT  TestAccAWSDBClusterParameterGroup_generatedName_Parameter
=== CONT  TestAccAWSDBClusterParameterGroup_generatedName
=== CONT  TestAccAWSDBClusterParameterGroup_namePrefix
=== CONT  TestAccAWSDBClusterParameterGroup_namePrefix_Parameter
=== CONT  TestAccAWSDBClusterParameterGroup_only
=== CONT  TestAccAWSDBClusterParameterGroup_disappears
=== CONT  TestAccAWSDBClusterParameterGroup_withApplyMethod
--- PASS: TestAccAWSDBClusterParameterGroup_disappears (28.27s)
--- PASS: TestAccAWSDBClusterParameterGroup_namePrefix (37.64s)
--- PASS: TestAccAWSDBClusterParameterGroup_only (37.73s)
--- PASS: TestAccAWSDBClusterParameterGroup_generatedName (37.78s)
--- PASS: TestAccAWSDBClusterParameterGroup_namePrefix_Parameter (38.05s)
--- PASS: TestAccAWSDBClusterParameterGroup_generatedName_Parameter (39.10s)
--- PASS: TestAccAWSDBClusterParameterGroup_withApplyMethod (39.17s)
--- PASS: TestAccAWSDBClusterParameterGroup_basic (98.63s)
PASS
ok      github.com/terraform-providers/terraform-provider-aws/aws       99.642s

[bflad]

LGTM! 🚀 (also should add bug label to PR 😉 )

Output from acceptance testing:

--- PASS: TestAccAWSDBClusterParameterGroup_basic (79.76s)
--- PASS: TestAccAWSDBClusterParameterGroup_disappears (11.95s)
--- PASS: TestAccAWSDBClusterParameterGroup_generatedName (32.92s)
--- PASS: TestAccAWSDBClusterParameterGroup_generatedName_Parameter (40.93s)
--- PASS: TestAccAWSDBClusterParameterGroup_namePrefix (63.28s)
--- PASS: TestAccAWSDBClusterParameterGroup_namePrefix_Parameter (120.10s)
--- PASS: TestAccAWSDBClusterParameterGroup_only (29.73s)
--- PASS: TestAccAWSDBClusterParameterGroup_withApplyMethod (56.26s)

--- PASS: TestAccAWSDBParameterGroup_basic (46.97s)
--- PASS: TestAccAWSDBParameterGroup_Disappears (11.44s)
--- PASS: TestAccAWSDBParameterGroup_generatedName (23.11s)
--- PASS: TestAccAWSDBParameterGroup_limit (72.95s)
--- PASS: TestAccAWSDBParameterGroup_MatchDefault (55.74s)
--- PASS: TestAccAWSDBParameterGroup_namePrefix (46.62s)
--- PASS: TestAccAWSDBParameterGroup_Only (37.66s)
--- PASS: TestAccAWSDBParameterGroup_withApplyMethod (51.45s)

[ghost]

This has been released in version 2.45.0 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!