provider/aws: Add support for IAMDatabaseAuthenticationEnabled

[gazoakley]

Adds a new iam_database_authentication_enabled attribute to aws_db_instance and aws_rds_cluster. Addresses #14069

[gazoakley]

Acceptance tests passing:

gaz$ make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSDBInstance_iamAuth'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/30 08:43:24 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSDBInstance_iamAuth -timeout 120m
=== RUN   TestAccAWSDBInstance_iamAuth
--- PASS: TestAccAWSDBInstance_iamAuth (491.42s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	491.439s

gaz$ make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSRDSCluster_iamAuth'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/30 08:58:45 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSRDSCluster_iamAuth -timeout 120m
=== RUN   TestAccAWSRDSCluster_iamAuth
--- PASS: TestAccAWSRDSCluster_iamAuth (134.36s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	134.380s

[grubernaut]

LGTM, thanks!

[ghost]

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