Cloned from launchpad blueprint https://blueprints.launchpad.net/keystone/+spec/use-common-oslo-db-code.

Description:

There is common code for work with database in Oslo. It contains work around session, SQLAchemy engines, wrappers, etc.
We can use this code instead of Keystone implementation.

So we should:
1) Sync common code for work with database from Oslo
2) Use Oslo code instead of current project implementation.
   - work with sqlalchemy session and engines
   - use sqlalchemy migration utils, to make work with migrations more clear

Specification URL (additional information):

None