Tweaks to make install run idempotent

roles/horizon/templates/horizon.service

@@ -12,8 +12,9 @@ ExecStart=/usr/bin/docker run --name %n \
   -e "KEYSTONE_HOST={{ hostvars[groups['keystone'][0]]['ansible_default_ipv4']['address'] }}" \
   --add-host=nova-controller:{{ hostvars[groups['nova-controller'][0]]['ansible_default_ipv4']['address'] }} \
   --add-host=keystone:{{ hostvars[groups['keystone'][0]]['ansible_default_ipv4']['address'] }} \
+  --add-host=neutron-controller:{{ hostvars[groups['neutron-controller'][0]]['ansible_default_ipv4']['address'] }} \
   -p 80:80 \
-  hpc/openstack-horizon
+  {{ docker_image }}
 
 [Install]
 WantedBy=multi-user.target