core: Introduce FencingPolicy

Adds FencingPolicy as a part of cluster configuration. The 1st option
added to FencingPolicy is the ability to skip fencing of host if it's
connected to at least one storage domain

Change-Id: I7dfcd571b8329174f8d3acccf7136d0b5f83cacc
Bug-Url: https://bugzilla.redhat.com/1090799
Signed-off-by: Martin Perina <mperina@redhat.com>