Allow ruby bin path to be configurable for chef zero

Inside lib/kitchen/provisioner/chef_zero.rb , the ruby path is hard coded to ominubus embedded ruby path. 
This should be configurable because in many cases ruby is installed by rbenv or such other tools.

This pull request provide rub_bin configuration option, for example.

```ruby
driver_config:
  require_chef_omnibus: false
  ruby_bin: "/usr/local/bin"
```