core: skipping DOWN hosts in PM proxy selection

Hosts with DOWN state were not filtered out when no UP Host available
and we are looking for Hosts in other statuses.
This patch addresses this issue and filter out all Hosts with DOWN
status.

Change-Id: Ie12002cbdbf025d5e53b2283dee7b373dac2a0a4
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1073896
Signed-off-by: Eli Mesika <emesika@redhat.com>