[skip ci] Update dashboards for RabbitMQ 4.1

Key changes:
- endpoint variable to handle scraping multiple endpoints
- message size panels (new metric in 4.1)
- panels at the top of the Overview dashboard should be more up to date
  (they show the latest value)
- values should be accurate if multiple endpoints are scraped
  (previously, many would be doubled)
- Nodes table shows fewer volumns and shows node uptime

Author: mkuratczyk