[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"
+}
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