add service provider for OpenBSD

Adds a service provider for OpenBSD. This was split off of another pull request: #2177. This initial commit has the proposed logic in place. I copied the tests from the FreeBSD provider and fixed them, but that's not good enough--the tests don't yet cover what they need to. I'll be taking another pass at them, but I didn't want to wait on that to get the discussion started on the rest of the code.
So the `provides` syntax I mentioned should be good to go in master now, and I think we should switch to using that with a new openbsd service resource instead of adding `afters` to the the top-level service resource. 

We're on the final stretch before freezing for Chef 12, and I believe @lamont-granquist is heroically trying to get #1596 in, which is a related improvement. But through a quick perusal it doesn't look like it would change the benefit of adding a provides line besides that it looks like @lamont-granquist is adding some additional syntax.