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.
this is carbon copied from Chef::Provider::Service::Simple. I poked around the git history of mixlib-shellout a bit and only found some exceptions raised in the Windows stuff, e.g. https://github.com/opscode/mixlib-shellout/blob/master/lib/mixlib/shellout/windows/core_ext.rb#L175. I couldn't find a ticket or issue related to the failure. Maybe @sersut could. Perhaps we never filed an issue to fix that, and we should create one and then annotate these TODO comments.