Bug 7861: No ping response from FIP on 1st router when adding 2nd FIP

Bug 7861: No ping response from FIP on 1st router when adding 2nd FIP

Problem Description:
====================
Table Miss Entry for SNAT/DNAT in FIB Table (table=21->26 flow) is
overriting when multiple router is present instead of creating new entry
for each VPN Instance.

Each router is associated with unique external network and each external
network is associated with unique internet BGP VPN Instance.

Example topology:
router-1/external-network-1/bgp-vpn-1
router-2/external-network-2/bgp-vpn-2

Both router-1 and router-2 are presented on single compute node. In this
scenario router-2 ping traffic only is working as expected. Since table 21
doesn't have the table miss entry for router-1 with bgp-vpn-1 instance.

Solution:
=========

In ExternalNetworkListener while creating table miss entry for SNAT/DNAT
in table 21, added "vpn-id" attribute also in creating flow reference as
same as implemented in the class
SNATDefaultRouteProgrammer.buildDefNATFlowEntity(). After this code change
in table 21 miss entry for SNAT/DNAT is created for each bgp-vpn instance.

Change-Id: Iac909e628f1a4134526e6df0b640149c8d45cd20
Signed-off-by: karthikeyan <karthikeyan.k@altencalsoftlabs.com>