Allow separate listen port in haproxy-config templates

This allows a service to pass in a different listening port than haproxy
listens on for the service. This allows haproxy to live on the same
network interface as the services, and will be used by services in
subsequent changes.

Change-Id: Id1d2429a4c800628cb43e0fb7d2b69131e71ee1c
Implements: blueprint service-hostnames