Remove config directive that breaks 2.8.x

RabbitMQ 2.8.x doesn't like the 'disc' parameter in the
cluster_nodes list in /etc/rabbit/rabbitmq.config.  That's not
entirely obvious because the documentation on Rabbit's website
is not versioned, but Rabbit won't start up with it present.
This change reverts our config template to a format that's
2.8.x friendly.

Closes-bug: #1262872