x/crypto/ssh: public key authentication example

Fixes golang/go#13902.

Adds public key authentication to the
password authentication example.

Change-Id: I4af0ca627fb15b617cc1ba1c6e0954b013f4d94f
Reviewed-on: https://go-review.googlesource.com/29374
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>