tests: Add sweepers for aws_internet_gateway and aws_nat_gateway

[bflad]

Got bit by a lingering NAT Gateway this morning so adding test sweepers. ♻️

This appropriately tags all the existing aws_nat_gateway test resources, but only starts the tagging process for aws_internet_gateway (I don't have time right now to tag and acceptance test the other few dozen test aws_internet_gateway resources).

make testacc TEST=./aws TESTARGS='-run=TestAccAWS\(Internet\|Nat\)Gateway_'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -run=TestAccAWS\(Internet\|Nat\)Gateway_ -timeout 120m
=== RUN   TestAccAWSInternetGateway_importBasic
--- PASS: TestAccAWSInternetGateway_importBasic (37.75s)
=== RUN   TestAccAWSNatGateway_importBasic
--- PASS: TestAccAWSNatGateway_importBasic (198.62s)
=== RUN   TestAccAWSInternetGateway_basic
--- PASS: TestAccAWSInternetGateway_basic (66.11s)
=== RUN   TestAccAWSInternetGateway_delete
--- PASS: TestAccAWSInternetGateway_delete (48.25s)
=== RUN   TestAccAWSInternetGateway_tags
--- PASS: TestAccAWSInternetGateway_tags (52.73s)
=== RUN   TestAccAWSNatGateway_basic
--- PASS: TestAccAWSNatGateway_basic (218.29s)
=== RUN   TestAccAWSNatGateway_tags
--- PASS: TestAccAWSNatGateway_tags (203.83s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	825.628s

make sweep
WARNING: This will destroy infrastructure. Use only in development accounts.
go test ./... -v -sweep=us-east-1,us-west-2
?   	github.com/terraform-providers/terraform-provider-aws	[no test files]
2018/02/27 09:49:08 [DEBUG] Running Sweepers for region (us-east-1):
...
2018/02/27 09:55:52 Sweeper Tests ran:
	- aws_beanstalk_application
	- aws_subnet
	- aws_redshift_cluster
	- aws_iam_server_certificate
	- aws_dynamodb_table
	- aws_beanstalk_environment
	- aws_autoscaling_group
	- aws_nat_gateway
	- aws_vpc
	- aws_kms_key
	- aws_mq_broker
	- aws_db_option_group
	- aws_gamelift_alias
	- aws_gamelift_build
	- aws_security_group
	- aws_lambda_function
	- aws_network_acl
	- aws_db_instance
	- aws_vpn_gateway
	- aws_gamelift_fleet
	- aws_key_pair
	- aws_internet_gateway
	- aws_launch_configuration
	- aws_dax_cluster
ok  	github.com/terraform-providers/terraform-provider-aws/aws	404.036s

[radeksimko]

LGTM, I'm going to merge it straight away and kick off tests.

Thanks.

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. Thanks!