[luxembourg] Add temporary server nat-belval

Will be renamed to nat once nat-belval is working well enough

data/grid5000/sites/luxembourg/servers/nat-belval.json

+{
+  "kind": "virtual",
+  "network_adapters": {
+    "default": {
+      "ip": "172.16.191.201"
+    }
+  },
+  "type": "server",
+  "uid": "nat-belval"
+}
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