IPv6 FIB enteries not appearing if ExtNwt enabled

IPv6 FIB enteries not appearing if ExtNwt enabled

Issue:
======
In dualstack networks, IPv6 FIB entries are not
appearing if router gateway is set with external
network. Due to this issue L3 connectivity across
IPv6 network is failing.

Solution:
==========
This fix is addressing the following use cases to
work properly for IPv6 Internet Connection in dual
stack network.

(1) IPv6 Subnet is added/removed from the External Router

(2) Internet BGP-VPN is getting associated/disassociated
from External Network

(3) External router is getting added/removed from external
network via router-gw set/clear

Dependency:
===========
This issue fix is dependent on review [0]

[0] https://git.opendaylight.org/gerrit/#/c/75657/

Issue: NETVIRT-1417

Change-Id: Ibfe6c3225810f4d82d5a58145df7ab1bf5a9f209
Signed-off-by: Karthikeyan Krishnan <karthikeyangceb007@gmail.com>