Enable logon-as-service in windows_service (CHEF-4921).

Yeah, so I'm feeling like you factored code out of a larger grant_service_logon, but that there's a PolicyFile object lurking in here that we're likely going to want to use elsewhere some day. (not to be confused with the [Chef PolicyFile](https://github.com/opscode/chef-dk/blob/master/POLICYFILE_README.md) work).

But it could end up being a waste of time so not so important right now, you know?