6 stable gce metadata update

Hi Adam & Opscode, This pull request updates GCE components in Ohai 6-stable: 

1) continued compatibility with Google's [Transitioning to v1beta1](https://developers.google.com/compute/docs/metadata#transitioning) for retrieving metadata from GCE.

2) removes the GCEL platform from Ohai. Our developer pages https://developers.google.com/compute/docs/faq#gcel mention that GCEL images have been deprecated and will eventually be removed from GCE.