Add missing default user & pass for RabbitMQ (#6883)

frontend/public/json/rabbitmq.json

@@ -28,8 +28,8 @@
     }
   ],
   "default_credentials": {
-    "username": null,
-    "password": null
+    "username": "proxmox",
+    "password": "proxmox"
   },
   "notes": []
 }