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 agree with Tim. Define a GCEPersistentDisk.

I also believe that the general form of of this would be "ExternalVolume", not "PersistentDisk". It's the class of things that act like storage devices and need special instructions/drivers for mounting. GCE's Persistent Disk is just one such thing. Other cloud providers may offer extremly different storage interfaces.