core: Display warning when fencing is disabled in cluster

Displays warning during add/update cluster if fencing is disabled in
fencing policy of the cluster.

Change-Id: I4853b349571523fbb4dac54cae45d9d6a9df01f3
Bug-Url: https://bugzilla.redhat.com/1120858
Signed-off-by: Martin Perina <mperina@redhat.com>