Add support for IAM credentials

As mentioned in https://github.com/test-kitchen/kitchen-ec2/issues/55.

This change will add support for using IAM temporary credentials when creating EC2 instances. Currently the credentials will have to be set in the environment, while an EC2 instance that's set up with an IAM profile can fetch its temporary credentials from the metadata server.
Make all the tests pass
Merge branch 'daanemanz/add-support-for-iam-credentials'