Handle BGPVPN Instance Update/Swap Case Properly

Handle BGPVPN Instance Update/Swap Case Properly

Issue:
======
Current implementation of the VPN Instance update
is always doing swap (Remove Old VPN Interface
and Add New VPN Interface) for when ever there
is an update with VPN Instance for existing VPN
interface.

This logic is suitable for only on Router is
getting associated/disassociated with
external BGP-VPN.

When Internet BGP-VPN is attached to router
all private IPv6 subnet routes needs to be
created with VPN interface onto Internet BGPVPN.

Single external BGP-VPN instance can be associated
to maximum two routers. This feature is supported
starting from fluorine release. This feature also
not working with current implementation.

Updating VPN Interface with second VPN Instance
should not consider the VPN Swap for Internet
BGP-VPN update and dual router use cases.

Solution:
==========
This fix addresses the following use cases
properly for BGP-VPN Instance update.

1) External BGP-VPN Instance is associated to
single router or disassociated from single router

2) Internet BGP-VPN Instance is attached to
single router(router-gw) for external public network
access or removed internet BGP-VPN instance from
single router.

IPv6 Internet/Public Data path traffic should work
along with review[0]

[0]https://git.opendaylight.org/gerrit/#/c/75601/

Issue: NETVIRT-1418

Change-Id: I71b495e0098d798c5a4ca51f395b0be67d51b2fe
Signed-off-by: Karthikeyan Krishnan <karthikeyangceb007@gmail.com>