Pecan: fix quota management

Pecan: fix quota management

This patch fixes quota management APIs in the Pecan framework.
To this aim:

1) an ad-hoc pair of collection/item controllers are introduced
   for the quota resource; as the new controllers have been added
   in a separate module, the neutron.pecan_wsgi.controllers.utils
   module has been added as well for helpers, routines and classes
   used by all pecan controllers;
2) the quota API extension is made pecan-aware, meaning that it
   simply returns a Pecan controller instance rather than deferring
   the task to the startup process that builds controllers using the
   home-grown WSGI framework ext manager;
3) the quota resource is now "almost" a standard neutron resource;
   unfortunately since it does not yet have its own service plugin a
   special provision is made in the attribute population hook in
   order to ensure the object is loaded for allowing correct
   policy enforcement.
4) Functional tests for the quota controller have been added.

Closes-Bug: #1505843

Change-Id: I44a1fd73f678e493d5b1163e5f183d9efdc678ac