Don't remove cookbook path, just add to list berkshelf cookbook path

Don't remove cookbook path, just add to list berkshelf cookbook path.

The problem what I can have also own directories with cookbooks:

```ruby
config.vm.provision :chef_solo do |chef|
    chef.cookbooks_path = ["site-cookbooks", "cookbooks"]
end
```

berkshelf should add own directory to this list (shouldn't remove my list).