core: skip non-responding hosts in proxy selection.

non-responding host is selected as a proxy for fencing operations

This patch adds the host NonResponsive status as an illegal status for
the proxy selection algorithm.
Hosts that are in NonResponsive state have communication problems that
prevents them from being proxy candidates.

Change-Id: Idaa861b32b20cca43a7fe41f318c3e8bcec4c95d
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1145321
Signed-off-by: Eli Mesika <emesika@redhat.com>