OHAI-507: (rebased) GCE Ohai plugin update for GCE v1beta1 compatibility

This pull request updates GCE components in Ohai 6-stable:

1) continued compatibility with our [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 documentation](https://developers.google.com/compute/docs/faq#gcel) mention that GCEL images have been deprecated and will eventually be removed from GCE.