Updates to `kitchen init` to be non-interactive (add `--driver` flag), add subcommand support, and introduce `kitchen driver discover`.

Having a non-optional interactive command is disruptive, hard to script,
and a surprising usability experience. Frankly, this author cannot
remember why he added it in the first place (late night fun with
RubyGems??).

The --driver flag defaults to `kitchen-vagrant` and can take more than
one driver if required.

This pull request also adds a new subcommand, `kitchen driver discover` which
searches RubyGems for project names of the form 'kitchen-*'--similar to
the formerly interactive `kitchen init` command.