enable a config flag to disable vagrant-librarian-chef if needed

Provide a config flag to enable/disable the usage of the plugin.
Usefull when you have vagrant-berkshelf and vagrant-librarian-chef installed (use case: cookbook developper that want to support both tools)
This changes the semantics of when the plugin is enabled by default. I think `enabled` should just default to true, so the user will receive an explicit error message if the plugin is enabled and the Cheffile can't be found.