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.
Are you setting this both in initialize and in load_current_resource in case the `service_name` has changed?  it's unclear here which value init_command is going to be set to when. If we need both, we should change the variable name to be more specific, if we only need one, set it in initialize and forget it?