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
Refactor Instance#_check_active

Currently, Instance#_check_active both checks whether the server is
active and attaches the volume. This commit separates that logic.

blueprint stack-check
Change-Id: I7fe805efc089535667e8d3840d79ade07430040c
Merge "Refactor Instance#_check_active"
Merge "Use oslo db.migration script"