Allow to set chef-zero-host when using the Chef Zero provider

This fix introduces a new configuration option chef_zero_host for the
Chef Zero provider. Using this option, you can bind the Chef Zero
server to another address than localhost.

Being able to set the host is nice when using test kitchen to test cookbooks that create docker containers or VMs which in turn use chef to configure themselves. This way the "nested" Chef clients inside the containers or VMs can access Test Kitchen's Chef Zero server, too.

Example kitchen.yml:

    - name: 'ops_server'
      driver:
        network:
        -
          - 'private_network'
          - ip: '192.168.99.100'
      provisioner:
        chef_zero_host: '192.168.99.100'
        client_rb:
          chef_server_url: 'http://192.168.99.100:8889'