Handle VPN Instance Update for dual router case

Handle VPN Instance Update for dual router case

Problem:
========
VPN Instance update is not properly handled for
dual router use case where single vpn interface
is part of two router. This configuration is
existing in the dual stack network where V4
subnet is part of one router and V6 subnet is
part of different router.

Solution:
============
This patch is addressing the following VPN instance update
scenario.

+1st VPN Instance : router VPN or external BGP-VPN-1.
+2nd VPN Instance : router VPN or external BGP-VPN-2.
+3rd VPN Instance : Internet BGP-VPN(router-gw update/delete) for public
network access.

Issue: NETVIRT-1394

Change-Id: If489d9537a96fd4139d589b14f7a204a91515e00
Signed-off-by: Karthikeyan Krishnan <karthikeyangceb007@gmail.com>