Fixes `aws_cloudwatch_log_subscription_filter` acceptance tests that had been failing since mid December
```
$ make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSCloudwatchLogSubscriptionFilter_basic'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/01/30 16:00:05 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSCloudwatchLogSubscriptionFilter_basic -timeout 120m
=== RUN TestAccAWSCloudwatchLogSubscriptionFilter_basic
--- PASS: TestAccAWSCloudwatchLogSubscriptionFilter_basic (26.34s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 26.364s
```