provider/aws: fixing the aws_internet_gateway resource

The resource is build so it can attach and detach the Internet Gateway from a VPC, but as the schema has `Required` and `ForceNew` both set to `true` for the vpc_id field it will never use these capabilities the way they are meant to..

The tests didn't change, but if you run the tests without this PR you'll notice a new Internet Gateway is created. With this PR that no longer happens so all features of this resource are now being used properly.