Document priority level on Keystone notifications

In the notification docs, we never explicitly say that notification are
always set at the INFO level. This priority level is not configurable
by the end user through the keystone.conf and this change documents that
behavior in the event_notifications.rst docs.

Change-Id: I293db2b69a73a1b56fe0539e183ea13b87ce639a
Merge "Document priority level on Keystone notifications"