SANITY: After configuring NAPT, table 26 and table 46 are not programmed

General perfomance related changes for beryliium. As such NeutronRouterDpnsMap, which was used by the SNAT was not getting populated. So in this commit, the changes were made for REVERTING the behaviour of the code related SNAT to behave like the one before.

Also the DECOUPLING of the code in the VpnManager module, handling the
NatNeutronRouterModel, is started with this review. New Class
NatInterfaceStateChangeListner in NatModule will be taking care of this model.

Changes in the RouterInterfacesListner add():

1) RouterInterfaces map of the ODL-L3 VPN has been populated irrespective
of the InterfaceState check, as done in the earlier commits.
RouterInterfaces map will be used by the InterfaceStateChangeListner to
populate/change the NeutronRouterDpnMap accordingly, whenever InterfaceState changes

2) InterfaceState check has been reintroduced for populating the
NeutronRouterDpnMap.

Changes made int RouterInterfacesLisner remove():

1) Code for removing the entry of the RouterInterfaces map for the
corresponding deleted NeutronRouterInterfacesMap entry has been
introduced.

Changes in the InterfaceStateChangeListner add():

1) RouterInterfaces map is being querried from the ODL-L3 VPN.

Changes in the InterfaceStateChangeListner remove():

1) Code handling the removal of the Nat related NeutronRouterDpnMap has
been moved to the new Listener NatInterfaceStateChangeListner

Changes in VpnInterfaceManager :

1) One of the overloaded method removeFromNeutronRouterDpnsMap() has been
removed and the same code is introduced to NatInterfaceStateChangeListner