Use cache backend to store sessions, include Memcache with Horizon system

Change-Id: Id08a507ab16fd8b55c4468334e7921a8d8ac0412
diff --git a/horizon/server/cluster.yml b/horizon/server/cluster.yml
index fbe8df4..8ee48a3 100644
--- a/horizon/server/cluster.yml
+++ b/horizon/server/cluster.yml
@@ -1,5 +1,16 @@
 classes:
 - service.keepalived.cluster.single
-- service.horizon.server.single
+- service.horizon.server.cluster
 - service.haproxy.proxy.single
 - system.haproxy.proxy.listen.openstack.horizon
+- system.memcached.server.single
+parameters:
+  _param:
+    horizon_site_branding: "OpenStack Dashboard"
+  horizon:
+    server:
+      branding: ${_param:horizon_site_branding}
+      plugin: {}
+      session:
+        engine: "cache"
+