alpine: disable IPv6 automatic addresses

So we have predictable addresses if we ever want to firewall
individual hosts.

roles/alpine/templates/interfaces.j2

@@ -13,5 +13,6 @@ iface {{ iface.name }} inet static
     gateway {{ gateway | ipaddr('address') }}
 {% endif %}
 {% endfor %}
+    pre-up echo 0 > /proc/sys/net/ipv6/conf/$IFACE/autoconf
 
 {% endfor %}