Migrate to oslo.db

dtantsur: as with oslo.config, backward compatibility should be a concern
yuikotakada: Yeah, I suppose getting existing data from sqlite and convert it and then migrating it into oslo.db. How do you think?
dtantsur: I guess the easier path it so make sure that after upgrade new oslo.db backend is just using the same sqlite database. this requires:
 1. ensure that oslo.db and old db schemas are the same
 2. old 'database' configuration option value is handled properly
 Should be enough IMO.
yuikotakada: Ah! Then, how about this? migrate to oslo.db with sqlite as backend -> migrate from sqlite to mysql
dtantsur: the same as with other OpenStack projects, if I remember correctly there's no automation here. you stop the service, dump the database, load into mysql and start the service with new config. I don't think we should invent something here, unless oslo.db provides it.
yuikotakada: I will investigate oslo.db and some other projects which has done similar migration.

Gerrit topic: https://review.openstack.org/#q,topic:bp/migrate-to-oslodb,n,z

Addressed by: https://review.openstack.org/181905
    Migrate to oslo_db