guard_interpreter default change for powershell_script, batch resources

This change modifies the default for the `guard_interpreter` attribute for the `batch` and `powershell_script` resources so that for each of these it uses those resources to execute guard expressions by default. That is to say, for `powershell_script`, the `guard_interpreter_attribute` will be set to `:powershell_script` and for the `batch` resource it will be set to `:batch`. 

Maybe we can add an example here with a pure ruby guard like [this](https://github.com/opscode/chef/blob/master/spec/functional/resource/execute_spec.rb#L44-L52) ?