Adds Persistent Disk volume type.

Implements a generalized Persistent Disk resource with plug-able API and Mounter interfaces.

Currently only supports GCE API, but the Persistent Disk resource was made general enough to support additional APIs.




I don't think we want this abstraction yet.  We don't have any alternate implementations to inform the design.  everywhere that you use these functions elsewhere you know it is GCE, so the abstraction is not useful.