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 this isn't really what we wanted.  We've got Chef::Util::FileEdit already and this should get converted to use that for now.  Its been used and tested and works for what it does.  If it needs extensions to the API we could add some methods to that class to accomplish what you need.