Extract .kitchen.yml to an ERB template & update box URLs.

Closes #21
Fix :require_chef_omnibus driver_config option to eliminate re-installs.

There are 4 differnt use cases or behaviors:

* `true` - the latest release will be installed. Subsequent converges
  will skip re-installing if chef is present.
* `latest` - the latest release will be installed. Subsequent converges
  will always re-install even if chef is present.
* `<VERSION_STRING>` (ex: `10.24.0`) - the desired version string will
  be passed the the install.sh script. Subsequent converges will skip if
  the installed version and the desired version match.
* `false` or `nil` - no chef is installed.