Revert "Remove broken quota-classes API"

This reverts commit 7d22153d05c67bb0bf4838efdb17b3360ec2e367.

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

Conflicts:
	doc/api_samples/all_extensions/extensions-get-resp.json
	doc/api_samples/all_extensions/extensions-get-resp.xml
	nova/tests/integrated/api_samples/all_extensions/extensions-get-resp.xml.tpl

The conflicts are due to Mark McLoughlin's timestamp-format series
of changes on master (Juno).

Also note that quota_classes.py was changed due to
commit c75a15a4 to rename the NotAuthorized exception
to Forbidden.

Change-Id: I7890e6b41d4ebf19944c1d4db65111fcc4c45463
(cherry picked from commit 35338d54ad2f76807011a5b666df434eec5369b2)