pyroute2: DHCPv6 support

DHCPv6 support added to pyroute2 configurator.

Change-Id: I225ac28a3da0720acdbb534aff2089b04964bec6
Signed-off-by: Petr Horáček <phoracek@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/32451
Reviewed-by: Ondřej Svoboda <osvoboda@redhat.com>
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>