prepare-node-join: Swap private and public keys

Other places, such as “ssh.GetUserFiles”, use a structure where the
private key comes before the private key. Until now prepare-node-join
did the opposite, that is the public key came first. To avoid confusion
and potential bugs, this is changed.

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