Offer unused quota resources as revocable

Goal:

I have high-priority Spark jobs, and best-effort jobs. I need my high-priority jobs to pre-empt my best-effort jobs, so I'd like to launch the best-effort jobs on revocable resources.

Problem:

Revocable resources are currently only created via oversubscription, where resources allocated to but not used by a framework will be offered to other frameworks. This doesn't support the ability for a high-pri framework to start up and pre-empty a low-pri framework.

Solution:

Let's allow quota (and ideally any reserved resources) to be configurable to be offered as revocable resources to other frameworks that don't register with the role.