Add helper for accessing options in correct precedence
Fix :librarian configuration handling

Librarian-Chef was not run by default when SoloCook/Bootstrap was
invoked by for example `knife bootstrap --solo` (from #207) or `knife
digital_ocean droplet create --solo` (from knife-digital_ocean).

This is probably caused by a Knife/mixlib-cli issue about handling
boolean values, but we can work around it by using our own helper.