netgroup: avoid extraneous LDAP search when retrieving primary key from DN
    
    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>