Title: #109: SSSCTL: fix netgroup-show parsing
I found that `obj_type` is being provided as an argument to `sssctl_print_object()` and
re-worked this PR to take advantage of this. I hope it will be cleaner than sneaking the
objectClass into the attrs array and allow attr_fn functions to use `obj_type` it in the
future if necessary.
See the full comment at https://github.com/SSSD/sssd/pull/109#issuecomment-268060812