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 don't anticipate it to *not* be an int, but I wasn't sure if there was a strange partition-naming scheme I wasn't aware of. If it's standard to be an int, we can make it an int.