DNs for netgroup entries can contain either 'cn' or 'ipauniqueid' attribute in
their leaf RDN depending on their origin. Since 'cn' is the primary key, we
can return it in `get_primary_key_from_dn` right away and avoid any extraneous
LDAP search.
https://fedorahosted.org/freeipa/ticket/5855
Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>