api, iproute2, dhclient: Support DHCPv6

First, propagate 'dhcpv6' through add/setupNetworks.
The forgotten property can now reach ifcfg configurator, which has
already supported DHCPv6, and iproute2 configurator, which
(using dhclient) now learned DHCPv6 too.

DHCPv6-only setup needs dhclient 4.2.7-2.fc20, or BZ 1129500 fix
to be backported to RHEL 6 and 7 (this should happen shortly).

Change-Id: I40a3d5d63d14e6a1cb8a70894a63de3a3367b9e7
Signed-off-by: Ondřej Svoboda <osvoboda@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/30554
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>