tbtool: Support other authentication modes - other SSH keys, or passwords

[joerick]

As discussed here - http://forum.tingbot.com/topic/71 - tbtool currently only works with the bundled SSH key and doesn't try any other auth methods. If people want to secure their Tingbots, they need tbtool to look for other keys or offer a password.

If people would like this, please chime in here so I can prioritise!

[xtramural]

I think any effort that hardens the Tingbot would be a good move given the concerns that surround IoT devices.