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 vagrant-berkshelf has the same kind of logic, i.e. enabled by default if the config file is present and the config flag not specified.

But it's your plugin, then your choice :).