RabbitMQ CPU usage

[Christophe Le Guern]

- OS version: Queens
- Linux: Ubuntu Xenial
- RabbitMQ: 3.6.14-1

RabbitMQ erlang beam.smp process is using a lot of CPU by default, around 40 and 50% CPU.
The CPU usage decreases to <10% when adding the following configuration to "/etc/rabbitmq/rabbitmq-env.conf":
```
RABBITMQ_SERVER_ADDITIONAL_ERL_ARGS="+sbwt none"
```

RabbitMQ documentation link: https://www.rabbitmq.com/runtime.html#scheduler-wakeup-threshold

This configuration is already applied in TripleO https://docs.openstack.org/releasenotes/tripleo-heat-templates/queens.html#relnotes-8-1-0-stable-queens-other-notes

This option could be part and be the default to openstack-ansible-rabbitmq_server ansible role.