WIP: Persistent disks proposal

This is a draft proposal for expanding GCEPersistentDisk to a fuller PersistentDisk framework.
Yes, storage is a request and the "provider" that is running Kubernetes will provide the implementation of storage.  

The cloud provider implementation for GCE provides methods for creating/attaching disks.  AWS will need to be enhanced with the same.  Other implementations of "cloud provider" could be created for other environments.  Otherwise, a different "plugin" architecture could be defined for storage.