Have COLLECTOR_PORT be a configuration knob. #4432

This is designed for applications which are built as
wrappers around HTCondor and want to emulate the default port
behavior in the tools.
Examples are OSG's RSV and HTCondor-CE.

Currently, this is (purposely) undocumented as there's no use for
this config outside developers.
Code touch-up and documentation as identified in the review. #4432