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 honestly don't know - you're assuming that basename + partition == device file name, but I think that is a dubious assumption globally.  udev can do all sorts of crazy stuff.  It happens to work in our container VMs, but I wonder if it would work in a CoreOS node on GCE?