Nova always tries to create the rabbitmq user regardless of
whether RabbitMQ is enabled or not.
This ps also adds an external rabbitmq doc.

Change-Id: Iec517226e4c82ea351889b55689a3efceaadcc76

Adds rabbitmq_server_additional_erl_args variable which
is appended to RABBITMQ_SERVER_ADDITIONAL_ERL_ARGS
environment variable to RabbitMQ server startup script.

This can be used to configure the schedulers.

Docs attached.

Change-Id: Id683c8cc6dac61354ffd94f3b460335b42136ba2
Co-authored-by: Radosław Piliszek <radoslaw.piliszek@gmail.com>
Related-bug: #1846467