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.
Looks like we're repeating this pattern in many places. Can we have a helper function for it to reduce the duplication?

```
  files['/etc/rc.conf'].split("\n").each do |line|
    if line =~ /^#{Regexp.escape(var_name)}=(.*)/
      old_value = $1
      break
    end
  end
```