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`. 

Sort of, :default may not have been the best name in the original implementation. Really it means "default_shell" or "os_shell." It also happens to be the default for all Chef resources (i.e. /bin/sh on Unix, cmd.exe on Windows).

This particular change isn't really changing the default for all resources, just for powershell_script and batch. Possibly having something like ":self" might make it easier to distinguish between :default and the actual default for a given resource.