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.
Small gofmt update...
Merge pull request #667 from svanharmelen/f-fix-schema-aws-internet-gateway

provider/aws: fixing the aws_internet_gateway resource