CLOUDSTACK-2045:[Multiple IPs Per Nic] This feature is not working well

in case of networks with external devices after GC

adding missing 'retrun false' for isNetworkReadyForGc for the networks
that use external network devices and has secondary IP's associated with
nics.

server/src/com/cloud/network/NetworkModelImpl.java

@@ -2143,6 +2143,7 @@ public class NetworkModelImpl extends ManagerBase implements NetworkModel {
         // network rules. So prevent network GC.
         if (secondaryIps != null && !secondaryIps.isEmpty() &&
                 networkIsConfiguredForExternalNetworking(network.getDataCenterId(), networkId)) {
+            return false;
         }
 
         return true;