Use default netcfg when NETCFG_FILE is unspecified

Change-Id: Ia563390ad75d45be7f0f50fdf515a63d2984f37a
diff --git a/trellis/docker/entrypoint.sh b/trellis/docker/entrypoint.sh
index 8702949..0de0b94 100755
--- a/trellis/docker/entrypoint.sh
+++ b/trellis/docker/entrypoint.sh
@@ -26,6 +26,8 @@
         echo "${NETCFG} does not exist"
         exit 0
     fi
+else
+    onos-netcfg ${ONOS_IP} ${TOPO}.json || exit 0
 fi
 
 # Start mininet