Librarian-Chef is not run by default when SoloCook/Bootstrap invoked from ruby

When running e.g. `knife bootstrap --solo` (from #207) or `knife digital_ocean droplet create --solo` (from [knife-digital_ocean](https://github.com/rmoriz/knife-digital_ocean)), librarian-chef is not run as `config[:librarian]` is not set to its default value, `true`.

I'm quite sure this is connected with a Knife/mixlib-cli issue about handling boolean values. But anyway we have to fix it ourselves now.


<!---
@huboard:{"order":212.0}
-->