* provider/aws: Support kms_key_id for `aws_rds_cluster`
```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSRDSCluster_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSRDSCluster_
-timeout 120m
=== RUN TestAccAWSRDSCluster_basic
--- PASS: TestAccAWSRDSCluster_basic (127.57s)
=== RUN TestAccAWSRDSCluster_kmsKey
--- PASS: TestAccAWSRDSCluster_kmsKey (323.72s)
=== RUN TestAccAWSRDSCluster_encrypted
--- PASS: TestAccAWSRDSCluster_encrypted (173.25s)
=== RUN TestAccAWSRDSCluster_backupsUpdate
--- PASS: TestAccAWSRDSCluster_backupsUpdate (264.07s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 888.638s
```
* provider/aws: Add KMS Key ID to `aws_rds_cluster_instance`
```
```
Usage of a helper function was assuming that an error would be returned
in a not found condition, when in fact a nil pointer was
returned.
Attached test crashes w/o fix, passes with it.
Fixes#5350
Refs #5418
