core: Add option disable fencing in cluster

Adds option to cluster fencing policy which disabled fencing for all
hosts in cluster. By default fencing in cluster is enabled.

Change-Id: I587d7d40bc010196f916525ec3da7b1d821689d9
Bug-Url: https://bugzilla.redhat.com/1120858
Signed-off-by: Martin Perina <mperina@redhat.com>