RabbitQM vs RabbitMQ typo fix

Seems like a typo has been repeated all over the doc with MQ instead
of QM. This patch fixes it:
- rename of the file section_debconf-rabbitqm.xml into
section_debconf-rabbitmq.xml.
- rename of xml:id="debconf-rabbitqm" inside that file to
xml:id="debconf-rabbitmq".
- rename of all references to debconf-rabbitqm accordingly.

Thanks to Matt Kassawara who noticed it.

This is a backport of: https://review.openstack.org/#/c/98638/

Change-Id: I917a8a87eb8d95d521970a04725b5cabd5764588