Improve RabbitMQ performance by reducing ha replicas

Currently we do not follow the RabbitMQ advice on replicas here:
https://www.rabbitmq.com/ha.html#replication-factor

Here we reduce the number of replicas to n // 2 + 1 as advised
above. The hope it this helps speed up recovery from rabbit
issues.

Related-Bug: #1954925
Change-Id: Ib6bcb26c499c9884faa4a0cd51abaec00cacb096