The installation guide configures services to use the RabbitMQ 'guest' account. However, RabbitMQ version 3.3 or newer limits access by the 'guest' account to the server that runs RabbitMQ [1]. This limitation prevents access by services running on network and compute nodes. Most distributions supported by the installation guide for Juno currently include RabbitMQ versions older than 3.3 and therefore not impacted by this issue. However, distributions can update package versions at any time and this issue currently impacts anyone using packages directly from RabbitMQ.
One solution involves editing the RabbitMQ configuration file to enable remote access by the 'guest' account. However, testing this solution revealed that older versions of RabbitMQ don't understand the options and use different names for the configuration file. Furthermore, RabbitMQ doesn't make obtaining the version number particularly easy. A more ideal solution probably involves adding a non-guest account (e.g., 'openstack') to RabbitMQ and configuring services to use it.
[1] https://www.rabbitmq.com/blog/2014/04/02/breaking-things-with-rabbitmq-3-3/
Ubuntu 14.10 ships with rabbitmq-server 3.3.5-1 as of December 31, 2014. It would be a good idea to update teh docs directly to support creating an openstack-specific, non-guest user for rabbitmq during the installation process. This would match up with not using the default usernames for any other service installations.