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.
I need to think about it. Sometimes services need to be started in a certain order. When you hand edit the rc.conf.local file and update the pkg_scripts setting, you would make sure everything is in the right order. If Chef just appends the latest service to be enabled to the end of the list, unaware of any ordering requirements, its possible the ordering would be wrong...basically you would need to enable the services in the order you want them to start, and if a service is already enabled, you wouldn't be able to put a new service in front of it except by disabling the old service, enabling the new one, then enabling the old one again.