[louvain] Add description of backup gateway

data/grid5000/sites/louvain/servers/g5k-gw.json

+{
+  "kind": "physical",
+  "network_adapters": {
+    "default": {
+      "ip": "172.17.223.250",
+      "mac": "d8:eb:97:b3:aa:27"
+    }
+  },
+  "redfish": false,
+  "type": "server",
+  "uid": "g5k-gw"
+}
\ No newline at end of file

input/grid5000/sites/louvain/servers/physical.yaml

@@ -39,3 +39,15 @@ srv-data:
       mac: e8:eb:d3:30:2c:3e
     adm:
       ip: 172.17.223.3
+
+# This machine is used to access Louvain G5K Site
+# through UCL's network, in case of network outage.
+# It will be replaced by the network backup when site
+# installation is over.
+g5k-gw:
+  kind: physical
+  redfish: false
+  network_adapters:
+    default:
+      ip: 172.17.223.250
+      mac: d8:eb:97:b3:aa:27