Sync DB models and migrations in keystone.assignment.backends.sql

This patch syncs models with migrations for:

-Domain
-GroupDomainGrant
-GroupProjectGrant
-Role
-UserDomainGrant
-UserProjectGrant

No actual schema change is taking place, this patch just corrects errors
in the model definitions.

partially implements bp db-sync-models-with-migrations

Change-Id: Ib3f22f56a9d00c725f21036ab4bfe4df5fad6876
Merge "Sync DB models and migrations in keystone.assignment.backends.sql"