core: Suppress VdsNotResponding execution if fencing is disabled

Suppress VdsNotRespondingCommand execution if fencing is disabled in
fencing policy of the cluster.

Change-Id: I784564e41c89b4a95eff3377cd6545cf907ecdce
Bug-Url: https://bugzilla.redhat.com/1120858
Signed-off-by: Martin Perina <mperina@redhat.com>