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 many moons ago we added a resource lookup feature where you could add a line to a resource (e.g. openbsd_service) like this:
```
provides :service, :on_platforms => ["openbsd"]
```

The goal was to avoid adding more attributes to the core service resource that only one service provider used. Unfortunately we found this broke ChefSpec in #1773, but we're working on fixing that widely in Chef 12 in #2216, because we still think it's best to add provider specific resources to a single specific resource, rather than the top-level resource and cause confusion over what that attribute does on other providers. This fix is going to be completed soon,, so we should switch to the provides behavior here and we can take advantage of that fix.