Replace magic value 'service/security' in CadfNotificationWrapper

With the 0.4.1 version of pycadf released, we can remove the use
of a magic value in the _send_audit_notification function

Change-Id: I5b8c33e8867a65eda242a83601e1f58b85c98112
Closes-Bug: #1323341
Merge "Replace magic value 'service/security' in CadfNotificationWrapper"