Switch DB migration engine to Alembic

Please read similar similar blueprint in nova: https://blueprints.launchpad.net/nova/+spec/convert-to-alembic

In addition:
1) SQLAlchemy-migrate has several bugs related to sqllite backend. this project no longer maintained and doesn't accept bugfixes. Cause of this we have to use monkey patching and other workarounds to solve them in other projects.
2) It has compatibility issues with sqlalchemy >0.8.x (solved by a separate patch)

Ceilometer is currently relatively small and it should be easier to convert it to Alebmic migration engine. Such conversion will provide a good reference point for other projects that will soon follow.