Support libcloud > 0.15 'metadata' argument format on cloud/gce module

In apache-libcloud > 0.15, for at least the 'create_node' method, the format in which the 'metadata' argument is expect has changed. This PR adds support for the new format while keeping support for the all format by detecting the 'apache-libcloud' version at runtime.

Actual changes on last commit, the other commits are just general code cleanups.