Use Alembic instead of Sqlalchemy-migrate in Manila

- Can emit ALTER statements to a database in order to change
  the structure of tables and other constructs
- Provides a system whereby "migration scripts" may be constructed;
  each script indicates a particular series of steps that can "upgrade"
  a target database to a new version, and optionally a series of steps
  that can "downgrade" similarly, doing the same steps in reverse.
- Allows the scripts to execute in some sequential manner.

1. Add Alembic migrations support.
2. Move 001_manila_init.py migration to
manila/db/sqlalchemy/alembic/versions/162a3e673105_manila_init.py.
3. Remove manila/db/sqlalchemy/migrate_repo directory.
4. Fix unit tests.
5. Add ability to runtime updrade/downgrade db.

Implements bp alembic-instead-of-sqlalchemy-migrate

Change-Id: Iadc0d9596e826323ba19bd25be741c401b90b688