WIP: Persistent disks proposal

This is a draft proposal for expanding GCEPersistentDisk to a fuller PersistentDisk framework.
I don't think size and performance are the only intents:

* min acceptable size
* min acceptable iops
* min acceptable latency
* min durability guarantee
* exclusive or non-exclusive access (although this is something that can be orthogonal to intent and we can potentially leave out)

I think you could model local / remote as a durability guarantee - 0 if you cannot tolerate any node downtime, a larger number (measured in time units?) as the amount of time before you wish to be down before being brought up with a stale copy of your data.