add local non-SSL listener for web UI

The cli tool rabbitmqadmin, which is used by the rabbitmq_vhost
provider, by default connects to port 15672. Commit 7f5a21f changed the
previous behivour from always listening on port 15672 to not listening
on port 15672 when ssl was set, breaking rabbitmqadmin and the
rabbitmq_vhost provider as it had no way of knowing which port or
whether it needed to connect to the web UI over SSL or not.

Having it listening on localhost:15672 was the simplest solution.