Added basic EC2 IAM metadata support for S3

Hi all,

Here is a small patch to add basic support for getting the AWS credentials from the instance metadata using IAM roles. This will allow the minion's IAM role to control what it can access in S3 and salt doesn't have to worry about credentials at all.

Please let me know if there are any things that aren't to style or function. I also removed a few lines of un-used code (nothing major).

Thanks,
-A.