Replace FLAGS with cfg.CONF in db

Replace all the FLAGS with cfg.CONF in cinder/db
Large commit "https://review.openstack.org/31172" was split into several parts
Use the common pattern: CONF = cfg.CONF

Change-Id: Ibac0a4b233ba82e13e3a9bfb6bd3fd418cdab29f
Fixes: bug #1182037
Merge "Replace FLAGS with cfg.CONF in db"