[grenoble] Add backupg5k server

data/grid5000/sites/grenoble/servers/backupg5k.json

+{
+  "kind": "virtual",
+  "network_adapters": {
+    "default": {
+      "ip": "172.16.31.144"
+    }
+  },
+  "type": "server",
+  "uid": "backupg5k"
+}
\ No newline at end of file

input/grid5000/sites/grenoble/servers/virtual.yaml

@@ -87,3 +87,8 @@ guix:
   network_adapters:
     default:
       ip: 172.16.31.128
+backupg5k:
+  kind: virtual
+  network_adapters:
+    default:
+      ip: 172.16.31.144