As requested in #4822, add support for a KMS Key ID (ARN) for Db
Instance
```
make testacc TEST=./builtin/providers/aws
TESTARGS='-run=TestAccAWSDBInstance_kmsKey' 2>~/tf.log
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /vendor/)
TF_ACC=1 go test ./builtin/providers/aws -v
-run=TestAccAWSDBInstance_kmsKey -timeout 120m
=== RUN TestAccAWSDBInstance_basic
--- PASS: TestAccAWSDBInstance_basic (587.37s)
=== RUN TestAccAWSDBInstance_kmsKey
--- PASS: TestAccAWSDBInstance_kmsKey (625.31s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 1212.684s
```
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