Disable vagrant-berkshelf plugin by default...

...because test-kitchen itself already takes care of resolving the cookbook dependencies via `berks`.

This is required for situations where a user has vagrant-berkshelf installed. In this case it will automatically kick in if a Berksfile is present. In the best case it would be just double effort, but in can also cause test-kitchen to fail.

In combination with berkshelf/vagrant-berkshelf#248 this will fix the above mentioned issue in test-kitchen/test-kitchen#575

Considerations:

 * it will disable vagrant-berkshelf via the default `Vagrantfile.erb`
 * ~~for the sake of completeness I added a config option to re-enable vagrant-berkshelf if someone really wants that (even though I can't imagine a good use case for it)~~
 * added a `Vagrant.has_plugin('vagrant-berkshelf')` guard so vagrant won't bail out if the plugin is actually not installed