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.




Comment should give an example.  e.g. is "ext3" a valid string here?  What types are supported?  I assume it depends on what the minions support, so we should say that.