Stale 21->snat_group flow for external subnet

Stale 21->snat_group flow for external subnet

Description:
Its been observed that, even on clean-up on External-network Suite,
few of the stale flows 21->snat_group for external subnet where not
been deleted. As a result, Bundle Resync test-case getting failed
with rersync flag enabled due to these stale flows pointing to an
snat_group with corresponding group missing.

Changes are done to take care of clearing vpn-instance-op-data DS
for external-fixed-ip properly ersutling in triggering
ExternalSubnetVpnInstanceListener
and take care of clearing these stale-flows.

Issue : NETVIRT-1311

Change-Id: I773a2f9935a600b7710796d21d9ce39cf4b9a8c0
Signed-off-by: xcheara <chetan.arakere@altencalsoftlabs.com>
Signed-off-by: shaik <shaik.b@altencalsoftlabs.com>