Allow configuration for cookbooks and site-cookbooks paths

In our chef-repo we want to be able to keep all our cookbooks under `site-cookbooks/` and have one `Cheffile` to populate the `cookbooks` directory, but have a separate test-kitchen setup for each cookbook. Allowing the cookbooks and site-cookbooks path allows us to achieve this.

For example, my configuration would look something like this:

```
# cookbooks/something/.kitchen.yml
provisioner:
  ...
  cookbooks_path: <%= File.join(ENV['CHEF_REPO'], 'cookbooks') %>
  site_cookbooks_path: <%= File.join(ENV['CHEF_REPO'], 'site-cookbooks') %>
```