Fix alternative config filename

This patch uses the rabbitmq.config attribute to name the config file and explicitly exports the corresponding environment variable in /etc/default.  We use this to run a separate rabbitmq service on a server that has the omnibus chef-server installed. (FYI, the omnibus rabbitmq mistakenly picks up the default config file in /etc/rabbitmq/rabbitmq.config, though this patch is still useful even if that is fixed).