Allow host port configuration

[ygina]

Two parties contact the host service: the controller and module requests.

The controller identifies the host IP/port based on the local IP address and port defined by the host in the register request. Thus the controller should be able to contact the host as long as the host is on the local network and it has a static IP. The scheduler should be able to handle if the host IP changes, but this could be a separate issue.

The module SDK hardcodes port 59583 as the location of host (the IP is always the local loopback interface). A potential issue is that if the user runs multiple hosts on the same computer, then the module code only knows one port, but I think it's reasonable to assume only one host per computer that can concurrently handle multiple requests. Another is if the user is running a different service on this port. The host should be able to configure which port the module reaches out to based on its own configuration, such as through an environment variable. The host should also create a custom firewall (see data/karl.net) based on this port.