setup-ssh: redo the logging levels

There is not enough logged by default on stderr, and way too much
information in the log file.

Since we don't want to debug paramiko itself, we filter only warnings
and above.

Otherwise, we make WARNINGS the default level, with verbose meaning INFO
and debug, well, DEBUG.

Additionally, a message is improved.

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