WIP: Persistent disks proposal

This is a draft proposal for expanding GCEPersistentDisk to a fuller PersistentDisk framework.
That was a suggestion only after thinking through what a NodeLocalDisk would look like (as opposed to NodePersistentDisk, I will rename).

If every pod gets EmptyDir by default, would NodeLocalDisk (persistence bool = false) achieve the same thing?  Flip the bool to true and suddenly NodeLocalDisk has local persistent storage (but without high availability).

Likewise for HostDir?  If you know the path you want for HostDir for some kind of existing data,  I presume you know the host, too.  Is this NodeLocalDisk that is persistent and has a HostName?

Introducing NodeLocalDisk made me think about these *Dir types.