Revert "Remove quota-class logic from context and make unit tests pass"

This reverts commit e25316ecb9c879cd1fe2de0c8a4c4d5a424324ab.

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: If903863d28fbab74cfe571709a01cd2cdd46d174
Merge "Revert "Remove quota-class logic from context and make unit tests pass""