Scale-in/Scale-out for NAT.

Scale-in/Scale-out for NAT.

Description
-----------

1) NatScaleIn added to handle remove event which is
registered as a callback on if the node not unmarked as tomstone.
Whenever Scale-in of an particular DPN fails and TOMSTONE=true entry been
removed or marked as false, if that DPN have some
Router Presence(with SNAT enabled) and if the corresponding NAPT switch is
missing, this DPN will be re-elected as NAPT switch.

2) New Listener - NatTepChangeLister is added to handle remove event. The
Existing logic of NatTunnelInterfaceStateListener.remove() been moved into
this Listener. Whenever the TEP on a given DPNID is removed, this API take
care of withdrawing the FIB entries for those Floating-IP existing on this
DPN and perform re-election of NAPT Switch for a VRF to which the current
DPN is elected as NAPT Switch.

3) Changes done to validate the candidate list of DPNs set with
TOMSTONE=true and exclude them during NAPT Switch election.

Change-Id: I70704c985c3d92078fee80a19b4311de8258f834
Signed-off-by: Chetan Arakere Gowdru <chetan.arakere@altencalsoftlabs.com>