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 Eric's point is valid (and I think we discussed it once before) - what happens when you put a PD volume in a replication-controlled pod template?  We need to define the semantics, even if the definition is "it's an error".  And then we need to defend against that happening.