hooks: Add a hook to configure IPv6 networking through custom properties

The hook forwards 'ipv6addr', 'ipv6gateway', 'ipv6autoconf' and 'dhcpv6'
properties to the API.

Change-Id: Ia0e427b182a1742999eeb14e004209b511668ba2
Signed-off-by: Ondřej Svoboda <osvoboda@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34571
Reviewed-by: Dan Kenigsberg <danken@redhat.com>