Fallback to Quota Conf Driver if Quotas table is not defined

Fallback to Quota Conf Driver if Quotas table is not defined

commit de15e0b9c5 enabled Quota DB driver default considering
production environments, but it breaks plugins without per-tenant
quota extension. In these plugin quotas tables is not loaded.

This commit fallbacks to ConfDriver if Quota model is not loaded by
checking neutron.db.quota_db which defines Quota model is imported.

Change-Id: Idaaaa9810598cfd3e5ce70020f498643b4819d16
Closes-Bug: #1236993
(cherry picked from commit 46495f4995b6604cb249b688aaa249ef4e8b18ef)