setup-ssh: fix the logging error message

This took a bit of interesting debugging, but the issue seems to be that
on older python/logging/paramiko (I'm not sure which is at fault here),
the other threads are finishing later than the main thread, so the
logging module closes the logging file (but doesn't unregister it)
before the paramiko thread(s) get a change to finish their logging.

Tested on both old versions (Dapper) and new versions (Lucid) of these
three pieces of the puzzle.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>