Support using AWS IAM Roles

This adds support for setting a config option(aws_iam_role) and passing along the correct value to fog.

It takes advantage of IAM Roles(http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/iam-roles-for-amazon-ec2.html) which are a way to give instances access to resources without storing your credentials on your instances.

I've got a copy of the AWS integration test with the only change being the settings and it works.  I can commit that as well, but since it only runs from an instance with an IAM Role I didn't think it made sense to be a regular spec.