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.




If it is not mounted, and we are calling DetachDisk, then it is safe to remove and detach the disk. Do you know an easy way to detect if the directory is not mounted other than scanning /proc/mounts?