core: Add fencing policy as parameter to fenceNode VDSM verb

Add fencing policy as a parameter to fenceNode VDSM verb. This policy
will be used during VdsNotResponding handling. Currently fencing policy
can contain attribute to skip fencing if host has live connection to at
least on of its storage domains. If turned on (default in 3.5 clusters)
and host has live connection to storage, fencing will be skipped and
info is displayed.

Change-Id: I61ca142f0630ec2d8c1ec16e44c01af989c19079
Bug-Url: https://bugzilla.redhat.com/1090799
Signed-off-by: Martin Perina <mperina@redhat.com>