Sync code with oslo-incubator

Updated openstack-common.conf and remove outdated modules. Ran update script
in oslo-incubator to generate modules in the mistral/openstack and tools
directories. Sqlalchemy backend has changed significantly in oslo-incubator.
Refactored mistral db api to address these changes and updated the base test
class DbTestCase accordingly.

Change-Id: I61327c33c0c8bc616af5e7633634461a2d7ac691
Implements: blueprint mistral-sync-with-incubator
Merge "Sync code with oslo-incubator"