Napt Switch Over not happening

Napt Switch Over not happening

Description : With using heat template for clean-up, it's been observed
that when the interface is removed from router north-bound event is
received, the ietf-interface entry in config DS been already deleted. As a
result, the router-interface remove event returns with only deletion of
interface-to-router mapping and not deleting any entry from
neutron-router-dpns(as unable to obtained dpn for the interface).

When interface-state south bound event is triggered, interface-to-router
mapping is already deleted and this event also returns without deleting
entry from neutron-router-dpns. As a result, the earlier continues to act
as napt switch for a given router even through it doesn't has any router
presence.

Changes are done to prevent immediate deletion of interface-to-router
mapping during north-bound event, so that this interface-to-router data is
available during south-bound event resulting in proper deletion of
neutron-router-dpns entry and triggers Napt Switch Re-election.

Issue : NETVIRT-1614

Change-Id: Ib9e8a6ea2b59ad073b9e6e3551bd6df1bd44fd96
Signed-off-by: Chetan Arakere Gowdru <chetan.arakere@altencalsoftlabs.com>