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.



I'm not sure about the factoring here. I feel like kubelet should make one cloudprovider object when it starts, based on a command line flag, not attempt to make them on demand. As written, this will try to mount a GCE pd even if we're running in vagrant or azure, which doesn't make any sense to me.