fencing: Skip fencing if host is maintaining its lease

1) Adds fencingPolicy parameter to fenceNode API call. The parameter is
   optional, but it's supposed to be used in oVirt >= 3.5.
2) fencingPolicy parameter can contain storageDomainHostIdMap, which
   is a map of storage domains and host ids
3) If clusterlock reports that host is live on at least one of specified
   storage domains, the fence agent execution is skipped and host is
   considered alive

Change-Id: I7b1fd9521cebea28d0402e53d46d74b95e73f383
Bug-Url: https://bugzilla.redhat.com/1090799
Signed-off-by: Martin Perina <mperina@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/30762
Reviewed-by: Oved Ourfali <oourfali@redhat.com>
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>