Use oslo db.migration script

Turn Heat's migration script into a simple wrapper around olso's.
The api is a bit different so for now keep ours inplace.
All that our script does is find the path to the migration_repo
and call the oslo code. No point having two implementations in the
same codebase.

Change-Id: I0aa3e10890f924d77268f4118dae53b446e193eb
Merge "Use oslo db.migration script"