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.
@rc_conf_local should be made an attr_accessor and then most of the ivar @-references to it should be dropped (outside of the setting in the constructor).

a few lines before this there's also a `@new_resource.service_name` reference which should have the @ dropped off of it.  for getters its better not to go directly through the ivar since if you typo an ivar you'll autovivify the typo-name while if you typo a reader you'll get a "NoSuchMethod" exception.