Adds ability to configure chef destination directory (/etc/chef)

In my company we use both Chef-client and Chef-solo to provision machines. 
Both are using the `/etc/chef` directory to store cookbooks, roles etc.

By default Gusteau overwrites the `/etc/chef`, and therefore prevents Chef-client from running.

This pull request allows to run chef-solo with a config directory other than `/etc/chef`
You can specify a custom target directory in `.gusteau.yml`:
 ```
 chef_config_dir: /etc/custom_chef_dir
 ```