BF+RF: make find_username capable of consulting SSH "internally"
Previous changes added explicit argument ssh_config, but it was provided only from 1 invocation points out of 3. As a result it caused to a crash, e.g.
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1065203 regression: TypeError: find_username() missing 1 required positional argument: 'ssh_conf'
TODOs
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test locally