WIP: Persistent disks proposal

This is a draft proposal for expanding GCEPersistentDisk to a fuller PersistentDisk framework.
what are kubernetes requirements around HostDir, then?  It *can* be used to access existing data, can't it? I don't know the intent of HostDir.

what about security?  can two pods mount that same HostDir?    

If HostDir is existing config/data/whatever, is it kind of a known pet and, if so, should it be a named top level persistent type that K8s knows about?

Replacing Empty/HostDir with NodeLocalDisk was just a suggestion/thought that can easily be cut.