Use netgroup_find instead of netgroup_show to workaround IPA bug.

Patch fixes https://bugzilla.redhat.com/show_bug.cgi?id=2144724 which
depends on https://pagure.io/freeipa/issue/9284

.
Add comment why replacing `netgroup_show` with `netgroup_find`.

Signed-off-by: Denis Karpelevich <dkarpele@redhat.com>