Add mistakenly deleted params

Some parameters were mistakenly deleted in
https://gerrit.mcp.mirantis.com/#/c/29366/

Change-Id: I456fb98287d4bcd52f9a9c4176491c8ff7178116
Related-PROD: PROD-24486
diff --git a/salt/control/placement/openstack/small.yml b/salt/control/placement/openstack/small.yml
index 07c918e..83f2254 100644
--- a/salt/control/placement/openstack/small.yml
+++ b/salt/control/placement/openstack/small.yml
@@ -14,6 +14,11 @@
     openstack_message_queue_node03_hostname: msg03
     openstack_proxy_node01_hostname: prx01
     openstack_proxy_node02_hostname: prx02
+    salt_control_xenial_image_backend: /var/lib/libvirt/images/backends/xenial.qcow2
+    openstack_control_backend_image: ${_param:salt_control_xenial_image_backend}
+    openstack_database_backend_image: ${_param:salt_control_xenial_image_backend}
+    openstack_message_queue_backend_image: ${_param:salt_control_xenial_image_backend}
+    openstack_proxy_backend_image: ${_param:salt_control_xenial_image_backend}
   salt:
     control:
       cluster: