Update highly available RabbitMQ section

Edited note to:
1. include RabbitMQ version.
2. remove the statement that Pacemaker with DRDB is preferred.

Change-Id: I34e73d404b49b3286634a66cfe78826a9d4f25b4
Backport: none
Closes-bug: #1330738
Merge "Update highly available RabbitMQ section"