Add test for compute API os-quota-class-sets

This tests showing and updating the 'default' quota class, which is
really all this API is used for. It was removed in Icehouse in part to
not knowing it was used even though python-novaclient and Horizon were
using it. The removal was reverted and this change is to enforce what
the API is used for.

The extension only lives in the Nova v2 API, it was removed in the v3
API in Icehouse.

This change adds new quota client and test classes for the new API.

An external lock fixture is used in all compute API quota tests to
avoid introducing race bugs with the existing tenant-specific
os-quota-sets tests.

Closes-Bug: #1325727

Change-Id: Ib0cde08dfaa0f6a5e180d247864fb59d76eca903
Merge "Add test for compute API os-quota-class-sets"