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.




It should be an error to specify a read-write PD in a replication controller, and I think this should be checked during validation of the manifest and have the kubelet simply reject replication controller manifests with RW PDs.