Persistent disks proposal

A stake in the ground ... something to evolve into a working implementation.
there are 2 options for this ReadOnly bool and I was not sure where to put it.

If that bool remains on the top level PersistentDisk object, it would require 2 "disk" definitions:  1 for read/write and the other for read-only.  There would also be 2 pod definitions:  1 for use w/ read/write disk and one for read-only (can have many of these pods).

If that bool goes down the chain to the VolumeSource object, then there is just 1 "disk" definition.  There would still be 2 pod definitions.

I prefer the latter (putting that bool down below).