NETVIRT-1148:SNAT Traffic is not working with Dual Stack Router

NETVIRT-1148:SNAT Traffic is not working with Dual Stack Router

Problem Description:
======================
Existing IPv4 NAT functionality is not working with Dual Stack Router
configuration scenario. Packets are getting dropped at ODL Controller.

Solution Proposal:
==================
Due to IPv4 and IPv6 Subnets are part of the single SNAT router, ODL
controller is not able to handle IPv6 Subnet properly. Due to this reason
IPv4 SNAT traffics are not working. To address this issue have made the
changes in IPv4 NAT service code to skip the IPv6 subnet process for
NATing. Since IPv6 NAT is not required. All IPv6 address are GUA
address's.

Change-Id: I7be6a999542e156047368203eee8e865b2c2eaac
Signed-off-by: karthikeyan <karthikeyangceb007@gmail.com>