Added alembic migrations

Added alembic dependency to requirements
Created alembic config files

To create alembic migration:
$ cd ./ceilometer/storage/sqlalchemy/alembic
$ alembic revision -m "migration_description"

Related to blueprint convert-to-alembic

Change-Id: I59cf409106e0ba8bb819f7fc6aeddd0a837fee4c
Merge "Added alembic migrations"