Explicitly use authentication methods for Net::SSH.start().

Otherwise, Net::SSH.start() will *not* try password authentication if
a ssh key is available.

This solves issue #767.
Add option '--shortcircuit-postinstall' to build command.

This will short-circuit to postinstall. It is usefully for testing
you post-install scripts without running throug the whole, time-consuming
install process.
Add documentation for option '--shortcircuit-postinstall'.
Do not try to start the box if it is already running.