COOK-2829 (yet another)

This pull request addresses the [comment ](https://github.com/opscode-cookbooks/aws/pull/39#issuecomment-26062694) made by @billmoritz in PR #39.

It also keeps the cookbook backward compatible: aws keys should still be loaded from the databag if the corresponding attributes are set.
A warning is logged to recommend the users to use resource parameters instead of the databag.