Properly set memcache backend for Horizon
Properly set memcache backend for Horizon

The Horizon configuration was no longer using memcached as backend,
and it did not set any value for SESSION_ENGINE.

This fixes bz#1255369

Change-Id: Ia5e374898c76d8ecc0fb1027dbc10ce323fe47bb

fr34k8/packstack,mahak/packstack,stackforge/packstack,stackforge/packstack,openstack/packstack,mahak/packstack,mahak/packstack,fr34k8/packstack,fr34k8/packstack,openstack/packstack,openstack/packstack,stackforge/packstack