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)
The problem is that if the Cheffile isn't found, the user will just get the message "Librarian-Chef is disabled for this machine." and it won't be clear that it was automatically disabled because the Cheffile wasn't found.