Switch to MemcachedCache backend for horizon

PROD-33487

Change-Id: I725840b3b64ae836a1d615069c40600f9a79dd79
diff --git a/horizon/server/cluster.yml b/horizon/server/cluster.yml
index 837d9b5..c788c92 100644
--- a/horizon/server/cluster.yml
+++ b/horizon/server/cluster.yml
@@ -15,6 +15,8 @@
       branding: ${_param:horizon_site_branding}
       bind:
         port: 8078
+      cache:
+        backend: 'django.core.cache.backends.memcached.MemcachedCache'
       plugin: {}
       session:
         engine: "cache"