Remove deprecated 'rabbitmq_apt_packages' variable
Remove deprecated 'rabbitmq_apt_packages' variable

'rabbitmq_apt_packages' has been deprecated since Newton and can be
removed.

Change-Id: I20cfae2dfe2aa3c63fd9e10ccbc217b68b31fcbe

openstack/openstack-ansible-rabbitmq_server,openstack/openstack-ansible-rabbitmq_server