Fix horizon to use cache

Horizon was missing SESSION_ENGINE from it's conf which means it was not
making use of memcached.

Change-Id: I450aee05f59e344902f1e92d913f4c1ce9e8dcc6
Closes-Bug: 1630509

ansible/roles/horizon/templates/local_settings.j2

@@ -158,6 +158,8 @@ SECRET_KEY='{{ horizon_secret_key }}'
 #    },
 #}
 
+{% if groups['memcached'] | length > 0 and horizon_backend_database | bool == False %}
+SESSION_ENGINE = 'django.contrib.sessions.backends.cache'
 CACHES = {
     'default': {
         'BACKEND': 'django.core.cache.backends.memcached.MemcachedCache',
@@ -169,6 +171,7 @@ CACHES = {
 {%- endif %}
     }
 }
+{% endif %}
 
 # Send email to the console by default
 EMAIL_BACKEND = 'django.core.mail.backends.console.EmailBackend'