Fix advertised JNLP Port

As far as I can figure, the /TcpSlaveAgentListener/ endpoint headers
tell the slaves what the JNLP port is. When I'm running jenkins behind a
proxy (marathon using the mesos plugin, which bridged networking), even
though I was setting the advertised port per a thread[1], it was not
accurately being returned in the header. This patch fixes that.

[1] https://groups.google.com/d/topic/jenkins-mesos/qUrXpDY07TQ/discussion