use $SERVICE_HOST in multi-node doc

The change fix multi-node doc to use SERVICE_HOST.
It resolves duplicate IP address.

Closes-Bug: #1485159
Change-Id: If86393e9a37bcb911a9aa125829cd8ce684edd9f
diff --git a/doc/source/guides/multinode-lab.rst b/doc/source/guides/multinode-lab.rst
index 557b522..1530a84 100644
--- a/doc/source/guides/multinode-lab.rst
+++ b/doc/source/guides/multinode-lab.rst
@@ -175,12 +175,12 @@
     SERVICE_TOKEN=xyzpdqlazydog
     DATABASE_TYPE=mysql
     SERVICE_HOST=192.168.42.11
-    MYSQL_HOST=192.168.42.11
-    RABBIT_HOST=192.168.42.11
-    GLANCE_HOSTPORT=192.168.42.11:9292
+    MYSQL_HOST=$SERVICE_HOST
+    RABBIT_HOST=$SERVICE_HOST
+    GLANCE_HOSTPORT=$SERVICE_HOST:9292
     ENABLED_SERVICES=n-cpu,n-net,n-api-meta,c-vol
     NOVA_VNC_ENABLED=True
-    NOVNCPROXY_URL="http://192.168.42.11:6080/vnc_auto.html"
+    NOVNCPROXY_URL="http://$SERVICE_HOST:6080/vnc_auto.html"
     VNCSERVER_LISTEN=$HOST_IP
     VNCSERVER_PROXYCLIENT_ADDRESS=$VNCSERVER_LISTEN