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.




Should we fail validation of the persistent disk provider when the apiserver is not using a gce cloud_provider?  I read this as showing that we accept gce as a persistent disk storage when running on azure, etc.  Is this intended?  Should it be configuration to apiserver to state what types of persistent disk providers are valid for a given instance of apiserver?