This adds a horizon_listen_port option, which defaults to horizon_port
for backward compatibility.
This option allow the user to differentiate between the port the
service listens on, and the port the service is reachable on. This is
useful for external load balancers which live on the same host as the
service itself.
Change-Id: I1e47e9524fd9c41bbb2cd2fc80560e53d9296599
Implements: blueprint service-hostnames
3ee1a7ce3c