Fix -- Ceph in multinode gate

This fix aims to address the multinode gate issues for IPs on the
same subnet.

Change-Id: If158f78acf4d5e32eb495354307a0e3e5343ef6e

tools/gate/funcs/network.sh

@@ -76,6 +76,8 @@ function find_multi_subnet_range {
     if ! [ "$(ip_diff $X)" -eq "0" ]; then
       SUBMASK=$(((($X - 1 )) * 8))
       break
+    elif [ $X -eq "4" ]; then
+      SUBMASK=24
     fi
   done
   echo ${FIRST_IP_SUBNET%/*}/${SUBMASK}