[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