Upstream changed default quota_driver

For Neutron Havana-rc2, the default quota_driver has been changed
to a more sensible value. Also, Horizon (and possible others) now
expects this quota_driver and will show errors if set otherwise.

See also the upstream bug and change:
https://bugs.launchpad.net/neutron/+bug/1189671
https://review.openstack.org/#/c/50378/

Change-Id: I47bd2fe8aa29614f1eaf15b9a2063883bb17d0e6