`find_migrate_repo` improvement

* Added `repo_name` attribute with default value as `migrate_repo`. Allow
flexibly look for migrate repos with different names.
* Abridged logic of dependency from a module location.
* Moved to `common.utils`. Required to synchronize with
`openstack.common.db.sqlalchemy.migration`

Change-Id: I8aa364f1654fb92fba287cd406670b03f8ec3a54
Merge "`find_migrate_repo` improvement"