Accept Credentials from IAM

Fog has the capability to look up temporary credentials to gain access to AWS resources. This is especially useful when running `test-kitchen` on an EC2 instance as part of a CI setup, and you want the CI server to spin up instances in EC2. By using credentials from the IAM role, you don't have to specify the credentials explicitly, or through env vars.

The added method checks whether an IAM role has been configured for the local machine through a simple curl against an EC2 instance's metadata server. If this is not present, the command will return either a 404 (EC2 instance without IAM role configured) or 000 (no EC2 instance) status, making the AWS credentials mandatory.