Fixes: #7492
When we use the same IP Address, BGP ASN and VPN Type as an existing
aws_customer_gateway, terraform will take control of that gateway (not
import it!) and try and modify it. This could be very bad
There is a warning on the AWS documentation that one gateway of the same
parameters can be created, Terraform is now going to error if a gateway
of the same parameters is attempted to be created
```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSCustomerGateway_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/03/07 18:40:39 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSCustomerGateway_ -timeout 120m
=== RUN TestAccAWSCustomerGateway_importBasic
--- PASS: TestAccAWSCustomerGateway_importBasic (31.11s)
=== RUN TestAccAWSCustomerGateway_basic
--- PASS: TestAccAWSCustomerGateway_basic (68.72s)
=== RUN TestAccAWSCustomerGateway_similarAlreadyExists
--- PASS: TestAccAWSCustomerGateway_similarAlreadyExists (35.18s)
=== RUN TestAccAWSCustomerGateway_disappears
--- PASS: TestAccAWSCustomerGateway_disappears (25.13s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 160.172s
```
b5b53bc56a