Use Oslo.db migration

Changed imports from keystone.common.sql.migration to
keystone.openstack.common.db.sqlalchemy.migration.py.
Surrounding code adapted.

Change-Id: I0bdfc6d386d8b11f01da9061ffaf15462840822e
Blueprint: use-common-oslo-db-code
Use global to represent OS-TRUST:trust

When sending a notification a resource_type is required, such as a
'user', 'project', or 'OS-TRUST:trust'. This value is consistent between
operations so it can be stored in a class global variable.

Change-Id: I1a9e66330d980e13d306f4d6a6c0f248e75a858e
Merge "Use global to represent OS-TRUST:trust"
Merge "Use Oslo.db migration"