Change event type for identity trust notifications

Our identity trust notification event type do not match keystone
event type for these notifications. This patch fix it.

Change-Id: I73c4ce9334a26574f3b2c83244ea67b219bec920
Closes-bug: #1392326
(cherry picked from commit 31350c54cb8d56880cd0b72811c532d07f8f5a28)