Refactor: cts: Always use first node to detect systemd, etc.

It shouldn't matter which node we use, so I don't see how it makes any
sense to pick a random one. I'm not worried about distributing the load
or anything like that.

Also drop self._target. We use it only in self._discover().

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>