Added ability to launch EC2 instances with an IAM Instance Profile

Support was already there to launch instances using IAM role credentials (https://github.com/mitchellh/vagrant-aws/pull/41), but I needed a way to launch an EC2 instance that had an IAM instance profile itself.

The AWS API (and Fog) let you refer to an IAM Instance Profile with either an ARN or a name.  I added support for both.