resource/aws_rds_cluster_instance: Add retries for IAM eventual consistency

[bflad]

Found during acceptance testing:

--- FAIL: TestAccAWSRDSClusterInstance_withInstanceEnhancedMonitor (148.81s)
    testing.go:518: Step 0 error: Error applying: 1 error(s) occurred:
        
        * aws_rds_cluster_instance.cluster_instances: 1 error(s) occurred:
        
        * aws_rds_cluster_instance.cluster_instances: InvalidParameterValue: IAM role ARN value is invalid or does not include the required permissions for: ENHANCED_MONITORING

Tests

9 tests passed (all tests)
=== RUN   TestAccAWSRDSClusterInstance_generatedName
--- PASS: TestAccAWSRDSClusterInstance_generatedName (637.65s)
=== RUN   TestAccAWSRDSClusterInstance_namePrefix
--- PASS: TestAccAWSRDSClusterInstance_namePrefix (668.34s)
=== RUN   TestAccAWSRDSClusterInstance_withInstanceEnhancedMonitor
--- PASS: TestAccAWSRDSClusterInstance_withInstanceEnhancedMonitor (677.46s)
=== RUN   TestAccAWSRDSClusterInstance_importBasic
--- PASS: TestAccAWSRDSClusterInstance_importBasic (696.97s)
=== RUN   TestAccAWSRDSClusterInstance_disappears
--- PASS: TestAccAWSRDSClusterInstance_disappears (721.75s)
=== RUN   TestAccAWSRDSClusterInstance_az
--- PASS: TestAccAWSRDSClusterInstance_az (747.42s)
=== RUN   TestAccAWSRDSClusterInstance_kmsKey
--- PASS: TestAccAWSRDSClusterInstance_kmsKey (795.83s)
=== RUN   TestAccAWSRDSClusterInstance_withInstancePerformanceInsights
--- PASS: TestAccAWSRDSClusterInstance_withInstancePerformanceInsights (797.43s)
=== RUN   TestAccAWSRDSClusterInstance_basic
--- PASS: TestAccAWSRDSClusterInstance_basic (1401.86s)

[mbfrahry]

LGTM

[bflad]

This has been released in version 1.17.0 of the AWS provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

[ghost]

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