not overwrite cookbooks_path, just append

I want to use both berkshelf cookbooks_path and self configured cookbooks_path.

I tested it with this test branch. https://github.com/riywo/vagrant-berkshelf/tree/keep_cookbooks_path_test

Currently, this change is hard to write rspec because it is an integrated test and needs some helpers for Vagrantfile, like [this](https://github.com/mitchellh/vagrant/blob/master/test/unit/support/isolated_environment.rb). 

If you think the test is required, I can try to create some helpers for this tiny modification.

Thanks!