Add new-world Quota object

Adds a Quota object that can track reservations.  This allows us to be
more OOP-like.  No code uses this yet but a follow-up patch will fix a
bug where compute is querying the DB.

Related to blueprint unified-object-model

Change-Id: I9f435691f12c7c268f754bb23a6330a7a2c419e4
Merge "Add new-world Quota object"