Extra indexes cleanup

After all migrations database state has a difference with model state
in each dialect.

There is difference in models with database in indexes.
For some dialects indexes is skipped and for other extra indexes declared.

Dropped unique index:
- `source.id` for mysql.

Created index:
- `resource_user_id_project_id_key` for mysql and sqlite.

bp: ceilometer-db-sync-models-with-migrations

Change-Id: I1fbb6dcdf6c3c21075caeef5241f09122d5e34c1
Merge "Extra indexes cleanup"