Currently two IPv6 related tests ipv6_app_114 (FTP) and ipv6_app_124
(TFTP) fails due to disabled automatic conntrack helper, so fix it by
enabling it explicitly.
Fixes: PCF-840
References: PCF-671
Signed-off-by: Petr Štetiar <ynezz@true.cz>
(cherry picked from commit 4b76442e50b28ce19de1278c3d3451eaede7aa48)
(cherry picked from commit 1d6070b9acbe731273fcf3534e97fec7adc11a38)