Implement quotas for openstack and mock

This pull request implements quotas for openstack and mock (ovirt currently does not support quotas whatsoever).

Providers now have a `quotas` method that returns an array of Quotas.  A Quota consists of the following attributes: name, usage, limit, and unit.

In my mind, the first quota returned should be considered the primary.

I think this should fit Jarda's wireframes, if I understood them correctly; let me know if they do not.