Revert "Partially remove quota-class logic from nova.quotas"

This reverts commit 96c92249a6e48fa391361efd9cdee6eac50b653c.

The quota_classes API was used to set default quota values
so it shouldn't have been removed, so reverting a series
of changes that removed the API and it's internal code.

Related mailing list thread on the topic:

http://lists.openstack.org/pipermail/openstack-dev/2014-May/035383.html

Partial-Bug: #1299517

Change-Id: Iea445877c02205ad64ccbc3864712fd11ba65464
Merge "Revert "Partially remove quota-class logic from nova.quotas""