change default neutron quota driver

Fixes: bug 1280361

The default in Havana is no longer to use the config file backed quota
driver.  Instead, it is to use:

neutron.db.quota_db.DbQuotaDriver

This commit alters the attribute to use this driver by default.

Change-Id: I0bd2661f9a12cf82c113f91c41cba3ff3f1096a8
Merge "change default neutron quota driver"