Adds the flag `rabbitmq_ha_replica_count` to change how many different
nodes a queue should be mirrored across. If the value is not set, then
it defaults to "ha-mode":"all". This value is unset by default to avoid
any unexpected changes to the RabbitMQ definitions.json file, as that
would trigger an unexpected restart of RabbitMQ during the next deploy.

Change-Id: Iee98cd937197a73a3b04aa8501fa325e8ecfff24