Add ability to specify custom config template for kitchen init

This allows you to state configurations in `kitchen.yml.erb` to be used in the templates for `kitchen init`.

I wanted to introduce testing to many different cookbooks without having to edit the `.kitchen.yml` file each time to change the box, box url, or provider type.

The code will ascend the file path until it finds `kitchen.yml.erb` and uses it as the template for creating the `.kitchen.yml` file.