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
Merge "Fallback to Quota Conf Driver if Quotas table is not defined"