tests/resource/aws_transfer_server: Add sweeper
tests/resource/aws_transfer_server: Add sweeper

Transfer endpoints are costly if left behind.

Output from sweeper in AWS Commercial:

```console
$ go test ./aws -v -sweep=us-east-1,us-west-2 -sweep-run=aws_transfer_server -timeout 10h
2019/10/09 09:48:52 [DEBUG] Running Sweepers for region (us-east-1):
2019/10/09 09:48:55 Sweeper Tests ran:
	- aws_transfer_server
2019/10/09 09:48:55 [DEBUG] Running Sweepers for region (us-west-2):
2019/10/09 09:48:57 [INFO] Deleting Transfer Server: s-7eb2d7c3d1d342e5a
2019/10/09 09:48:58 [DEBUG] Waiting for state to become: [success]
2019/10/09 09:48:58 [INFO] Deleting Transfer Server: s-eb4eaedbf546445f9
2019/10/09 09:48:59 [DEBUG] Waiting for state to become: [success]
2019/10/09 09:48:59 Sweeper Tests ran:
	- aws_transfer_server
ok  	github.com/terraform-providers/terraform-provider-aws/aws	9.023s
```

Output from sweeper in AWS GovCloud (US):

```console
$ go test ./aws -v -sweep=us-gov-west-1 -sweep-run=aws_transfer_server -timeout 10h
2019/10/09 09:50:39 [DEBUG] Running Sweepers for region (us-gov-west-1):
2019/10/09 09:50:43 [WARN] Skipping Transfer Server sweep for us-gov-west-1: RequestError: send request failed
caused by: Post https://transfer.us-gov-west-1.amazonaws.com/: dial tcp: lookup transfer.us-gov-west-1.amazonaws.com: no such host
2019/10/09 09:50:43 Sweeper Tests ran:
	- aws_transfer_server
ok  	github.com/terraform-providers/terraform-provider-aws/aws	4.772s
```

terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws