WIP: Persistent disks proposal

This is a draft proposal for expanding GCEPersistentDisk to a fuller PersistentDisk framework.
Pods are 1:1 with volumes.  A ReplController w/ 5 replicas would make either 5 separate disks (1:1 to a pod) or share the 1 disk across many pods (1:1 to the PodTemplate).  That is why I think it can remain in the Volume section of a pod definition.  It is easy enough to change it to a reference (by way of selectors or a specific name), but it seems like an extra step than is necessary for a developer.


1.  Pod w/ storage request is posted to API server
2.  The REST implementation  (where 'business logic' exists, validation, saving, doing stuff) :
2a.  Validates
2b.  Creates the pod in etcd
2c.  Creates the volume, as necesary, in etcd
2.  Various watchers do their thing
3.   On pod delete, use the retention policy to delete the disk or not