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.



Is it really an error to use the cloudprovider module from a machine that is not running on that provider?  e.g. could kubecfg ever use cloudprovider?  If not, we should document it clearly.