Consistent data bag key names

The data bag keys we were using were called <code>username</code> and <code>apikey</code> which was inconsistent with what we were using everywhere else, which was <code>rackspace_username</code> and <code>rackspace_api_key</code>

This pull request both fixes the keys to be more consistent, and gives an error and exits the chef run if the user is still using the old keys.