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.




Originally, it was  a GCEPersistentDisk type, but then I generalized it thinking there's some common functionality that could be shared (I imagine the mounts/bind-mounting procedures wouldn't change much, for example). Maybe some input from other cloud providers could help here? 

I don't mind un-generalizing it to avoid shooting ourselves in the foot.