Revert "Remove quota_class db API calls"

This reverts commit 9c285c4bc777141edc80c6a40a3e1ff491aae217.

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: I69e374cd0363bb7481c5bc96f1ba6415ac50fa81
Merge "Revert "Remove quota_class db API calls""