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.




For n volumes that have not been deleted, there are n + 1 references. The global mount, and n bind mounts. The code above the check un-mounts the binds, so if there is only 1 reference left, it is the global mount.