Flesh out 'Configure RabbitMQ for HA queues' in HA guide

[Alexandra Settle]

Few TODOs that need attention in the 'Configure RabbitMQ for HA queues' section of the 'shared-message.rst' file:

[TODO: This section should begin with a brief mention
about what HA queues are and why they are valuable, etc]

[TODO: replace "currently" with specific release names]

[TODO: Does this list need to be updated? Perhaps we need a table
that shows each component and the earliest release that allows it
to work with HA queues.]

[TODO: Should the example instead use a minimum of three nodes?]

These require addressing as they have been there for ages. Reporting bug for more visibility.

[Frank Kloeker (f-kloeker)]

converted to https://storyboard.openstack.org/#!/story/2005582