On systems where ipv6 module is not loaded in kernel we need to avoid
usage of ip6tables. This patch reads
/proc/sys/net/ipv6/conf/default/disable_ipv6 file and if ipv6 is
disabled then ip6tables are not used in IptablesManager
Closes-Bug: #1352893
Change-Id: I07e5851aa25eb98b7a97dff86b9850475df85f64