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.




This restriction will present problems for replication.
Since k8s manages replication, maybe it needs to manage creation of PDs for pods too.  

Also, what happens with this change if a user starts two pods that need the same PD.  Does it always fail or does it succeed if both pods happen to land on the same VM?