core: Check host status is UP for HA reservation

Making sure only hosts in 'UP' status are used in the HA 
reservation calculation.

Change-Id: Ibfd8eb840d70bb0c2f345f04e328cccc5a92439f
Bug-Url: https://bugzilla.redhat.com/1057579
Signed-off-by: Kobi Ianko <kobi@redhat.com>