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 think it makes sense to have the scope of the resource before the resource itself in the directory path.  The paths for pod-local resources should probably have a pods/ dir inserted in them at the same level as the global/ dir. Right now, if you named a pod "global" it would create pod volume directories under ../global/volumes/..., right next to ../global/pd/.... It doesn't functionally cause any conflicts for the time being, but it is definitely confusing.