Currently, the process of enabling RabbitMQ HA with the variable
``om_enable_rabbitmq_high_availbility`` requires some manual steps to
migrate from transient to mirrored queues. In preparation for setting
this variable to ``True`` by default, this adds a precheck that will
fail if a system is currently running non-mirrored queues and
``om_enable_rabbitmq_high_availbility`` is set to ``True``.

Includes a helpful message informing the operator of their choice.
Either follow the manual procedure to migrate the queues described in
the docs, or set ``om_enable_rabbitmq_high_availbility`` to ``False``.

The RabbitMQ HA section of the reference docs is updated to include
these instructions.

Change-Id: Ic5e64998bd01923162204f7bb289cc110187feec