Adds Persistent Disk volume type.

Implements a GCEPersistentDisk volume type.

Persistent Disks can now be specified in pod manifests and will be mounted and exposed to containers.



Hmm, regarding my previous remark about being a plugin...

Do we want a global cloud provider for kubelet, which we have to pass around (or make global) or do we want to create objects and fail at each place we need them?  I prefer the former...