RFE: Use Alembic for database migrations

[Lance Bragstad]

Move from SQLALChemy-migrate to Alembic as the tool for maintaining SQL Repos.

[Lance Bragstad (lbragstad)]

Specification review: https://review.openstack.org/#/c/131531/
Specification doc: http://specs.openstack.org/openstack/keystone-specs/specs/keystone/backlog/alembic.html

[Lance Bragstad (lbragstad)]

Patches that work towards this initiative:

alembic initial support: https://review.openstack.org/150057
Do not add new 'db' command and subcommands for it: https://review.openstack.org/177219
Target Alembic for Liberty: https://review.openstack.org/177220
Use migration_cli for db migrations: https://review.openstack.org/147548

[Vishakha Agarwal (vishakha.agarwal)]

Now we have two commands that are related to migrations -- db_sync to migrate and db_version to show the current version. It works OK with sqlalchemy-migrate, but not with Alembic that we will be using. What needs to be done is to create a set of commands - "db upgrade/downgrade/stamp/revision/version". It will comply with alembic commands and with oslo.db migration_cli interface.