Code review comment for lp:~allenap/maas/narrow-port-range--bug-1352923

I had a quick chat with Gavin to answer some of my ignorant questions about this MP.

Q: Why do we need so many ports?!
A: Because each cluster is responsible for connecting to every regiond process. (each RPC endpoint is listed in JSON format if you hit <SERVER_URL>/MAAS/rpc/. (clusters periodically refresh this)

Q: Why can't the cluster just connect to any random regiond?
A: Because any arbitrary regiond needs to be able to use this control channel to talk to any arbitrary clusterd. Any individual regiond has no way to directly contact any arbitrary clusterd otherwise.

That being the case, I think this change looks good. I noted a minor issue with an 'except' clause below, and possibly a more robust test case.