OHAI-507: GCE Ohai plugin update for GCE v1beta1 compatibility

Moving from https://github.com/opscode/ohai/pull/191. @paulrossman, can you look over the re-integration of dmi? @danielsdeleo, thanks for supporting dmi.

I've tested this on gce -- I don't see the gce attributes unless I run chef-client with sudo. Is this ok? @paulrossman, would you expect execution of ohai without sudo to not be able to fetch gce attributes?