provider/aws: RDS Cluster additions

[stack72]

Adding backup_retention_period, preferred_backup_window and preferred_maintenance_window to RDS Cluster

Fixes #3755

make testacc TEST=./builtin/providers/aws TESTARGS='-run=RDSCluster_' 2>~/tf.log
go generate ./...
TF_ACC=1 go test ./builtin/providers/aws -v -run=RDSCluster_ -timeout 90m
=== RUN   TestAccAWSRDSCluster_basic
--- PASS: TestAccAWSRDSCluster_basic (112.39s)
=== RUN   TestAccAWSRDSCluster_backups
--- PASS: TestAccAWSRDSCluster_backups (111.10s)
=== RUN   TestAccAWSRDSCluster_backupsUpdate
--- PASS: TestAccAWSRDSCluster_backupsUpdate (125.92s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    349.436s

[catsby]

This is fine to leave, but fwiw anything with a Default specified will always return true for ok in d.GetOk

[catsby]

Left some feedback, otherwise looks good! Let me know what you think about those proposed changes

[stack72]

@catsby you are correct on these changes. I tend to bunch the tests too much. Just made the changes :)

Thanks for the useful feedback here. It's stuff I can use in other PRs

[catsby]

Thanks for following up there. Sorry for the run around, but let's ditch TestAccAWSRDSCluster_backups and just have the latter one TestAccAWSRDSCluster_backupsUpdate (sp). The first test case of the latter is identical to the only test case in the former, sound good?

[stack72]

makes sense - no need for a duplicate test :) Sorted now @catsby

[catsby]

Thanks!

[ghost]

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