Ensure ip6tables are used only if ipv6 is enabled in kernel

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

Conflicts:
	neutron/agent/l3_agent.py

Change-Id: I07e5851aa25eb98b7a97dff86b9850475df85f64
(cherry picked from commit b6e9922364fca4d8d141fbb2f27024f7db79ca9e)