Fixes `aws_rds_cluster_parameter_group` acceptance tests, which have been broken since aa8c2ac587
```
$ make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSDBClusterParameterGroupOnly'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/01/30 16:20:38 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSDBClusterParameterGroupOnly -timeout 120m
=== RUN TestAccAWSDBClusterParameterGroupOnly
--- PASS: TestAccAWSDBClusterParameterGroupOnly (15.26s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 15.282s
```
```
$ make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSDBClusterParameterGroup_basic'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/01/30 16:22:48 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSDBClusterParameterGroup_basic -timeout 120m
=== RUN TestAccAWSDBClusterParameterGroup_basic
--- PASS: TestAccAWSDBClusterParameterGroup_basic (29.48s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 29.510s
```
Jake Champlin