Make setup-ssh behave more friendly

So while testing my previous patch I run setup-ssh manually without
any arguments, and it did nothing (no complains, no messages,
anything). That was very surprising, so let's make it behave better if
no nodes are passed.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>