cmd/tsp-forwarder: add ListenAddr (obsolete collect-site)

[masiulaniec]

Add a ListenAddr setting that specifies the listen address for the built-in data point listener (this listener is currently implemented in cmd/collect-site).

The ListenAddr setting is the last thing that prevents tsp-controller from managing all details of the network topology. Currently, the operator must pass the listen port as a flag to collect-site, even though this port is already declared in /etc/tsp-controller/network.

This is particularly relevant on machines that try to run both opentsdb and tsp-aggregator: they both try to listen on :4242, but only one is allowed.

[masiulaniec]

Integrated.