Add info about RabbitMQ requirement on quickstart
Add info about RabbitMQ requirement on quickstart

Add info about hostname resolution requirement for RabbitMQ on
quickstart page, corresponding to the docimpact resulted from
a previous merged commit https://review.openstack.org/#/c/294713/.

Closes-Bug: #1559809

Change-Id: I671e7d897412859f606ea88e78ac63605bd8824b

dardelean/kolla-ansible,dardelean/kolla-ansible,dardelean/kolla-ansible