Add role assignment notifications for identity

Keystone added CADF notifications for role assignments in Juno.
We should be able to meter these events from Ceilometer.

Spec is here:
http://specs.openstack.org/openstack/keystone-specs/specs/juno/role-assignment-notifications.html

Impl is here: https://review.openstack.org/#/c/112204/ (merged)

Change-Id: I64e9b66081e3d453347541fd231da7d8e2598eb3
Merge "Add role assignment notifications for identity"