Add the missing hosts group for designate project

refer to https://github.com/openstack/kolla-ansible/blob/master/ansible/inventory/multinode#L586



Change-Id: I3901c5c920b75879439f4945da59ea4288dd57a8
Co-Authored-By: chenqiaomin <chen.qiaomin@99cloud.net>
Closes-Bug: #1736728

ansible/site.yml

@@ -569,6 +569,7 @@
     - designate-mdns
     - designate-worker
     - designate-sink
+    - designate-backend-bind9
   serial: '{{ serial|default("0") }}'
   roles:
     - { role: designate,