Fix IAM Role ARN if trying to run against GovCloud

[dynamike]

Up til now we've been hardcoding the AWS partition, which breaks when trying to run against GovCloud. This PR looks up the AWS partition based on the region you're passing in.

Tested by running

go run cmd/main.go --role admin --iam-user dynatest --profile=dynatest --account-id=<gov-account-id> --region=us-gov-west-1
go run cmd/main.go --role admin --iam-user dynatest --profile=dynatest --account-id=<commercial-account-id>

[cblkwell]

Why not always return partition.ID? Won't this always be either "aws" or "aws-gov"? If it errors out, isn't that a bad sign? I suspect this is a "Chas doesn't understand Go" problem, but I'm curious nevertheless.

[mdawn]

Suggested change

	func getPartition(region string) string {
	if partition, ok := endpoints.PartitionForRegion(endpoints.DefaultPartitions(), region); ok {
	return partition.ID()
	}
	return "aws"
	}
	func getPartition(region string) (string, error) {
	partition, ok := endpoints.PartitionForRegion(endpoints.DefaultPartitions(), region)
	if !ok {
	return "", fmt.Errorf("Error finding partition for region: %s", region)
	}
	return partition.ID(), nil
	}

[mdawn]

Suggested change

	getPartition(options.AwsRegion),
	partition,

Then add above something to call getPartition like:

partition, err := getPartition(options.AwsRegion)
	if err != nil {
		fmt.Println(err)
		fmt.Println("Using aws as partition")
		partition = "aws"
	}

If you want to avoid noping out of the process

[dynamike]

@mdawn Thanks for keeping my Go honest. I made the suggested changes.

@cblkwell I think defaulting to the "aws" partition was a premature, and if it can't find the partition then bail out.