Add a default rule for dhcpv6 traffic

Add a default rule for dhcpv6 traffic

Via bug https://bugs.launchpad.net/tripleo/+bug/1657108 we need
to zero out the default rules in /etc/sysconfig/ip{6}tables in
the image.
We have done this for ipv4, but when we will do it for ipv6 we
will also need to make sure we add a rule for dhcpv6 traffic
as it is shipped in the iptables rpm. (See
https://bugzilla.redhat.com/show_bug.cgi?id=1169036 for more info)

With this change we correctly get the rule present (aka the first
ACCEPT line. The second line is due to the stock ip6tables rule
I had in my testing):
[root@overcloud-controller-0 ~]# iptables -nvL |grep 546
[root@overcloud-controller-0 ~]# ip6tables -nvL |grep 546
    0     0 ACCEPT     udp      *      *       ::/0 fe80::/64 multiport dports 546 /* 004 accept ipv6 dhcpv6 ipv6 */ state NEW
    0     0 ACCEPT     udp      *      *       ::/0 fe80::/64 udp dpt:546 state NEW

Change-Id: If22080054b2b1fa7acfd101e8c34d2707e8e7864
Partial-Bug: #1657108