Add CADF notifications for role assignment create and delete

Assigning and removing roles from users|groups and projects|domains
should be an auditable event. Thus, we should start sending out
notifications when these events occur.

implements bp role-assignment-notifications
Co-Authored-By: David Stanek <dstanek@dstanek.com>
Change-Id: I62384dee4077cd29062e4b59adb4d5a38ad9234c
Merge "Add CADF notifications for role assignment create and delete"