NAT failure due to Napt Switch-Over not happened

NAT failure due to Napt Switch-Over not happened

Description :
Whenever a last VM part given VRF is deleted/migrated, it is required
to delete 'dpn-vpninterfaces-list' entry for given dpn-id from
'odl-l3vpn:neutron-router-dpns' Operational DS. This trigger
re-election of new Napt Switch which has VRF presence. But, when 2
VMs part of same VRF on a same DPN deleted simultaneously, then
there are chances that both of these event doesn't delete
'dpn-vpninterfaces-list' entry for the DPN which results in
non-triggering on new Napt Switch election. As a
result, the SNAT reverse traffic initiated from Non-Napt Switch
fail as FIB routes will not be available to carry back traffic
to non-napt switch.

Solution: Changes are done to make sure the 'dpn-vpninterfaces-list'
entry for DPN is deleted properly when last VM on that DPN is deleted
for a given VRF.

Issue : NETVIRT-1221

Change-Id: Idaebd8576762d12639d7bfbc26a89af94de4e991
Signed-off-by: xcheara <chetan.arakere@altencalsoftlabs.com>