Using model_query in the operation for retrieving tenant limits
will spare the need for explicit authorization check in the
quota controller. This is particularly relevant for the pecan
framework where every Neutron API call undergoes authZ checks
in the same pecan hook.
This patch will automatically adapt by eventuals changes
introducing "un-scoped" contexts.
Closes-bug: #1505406
Change-Id: I6952f5c85cd7fb0263789f768d23de3fe80b8183
24b482ac15