NETVIRT-1709: Unblock the netvirt CSIT NAT exception

NETVIRT-1709: Unblock the netvirt CSIT NAT exception

Issue:
======
https://logs.opendaylight.org/releng/vex-yul-odl-jenkins-1/netvirt-csit-1node-0cmb-1ctl-2cmp-openstack-rocky-dcgw-aluminium/199/robot-plugin/log_full.html.gz

[{'warnerr_list': [1], 'lines': ['2020-09-12T11:48:59,484 | ERROR |
NeutronPortChangeListener-0 | NeutronPortChangeListener | 360 -
org.opendaylight.netvirt.neutronvpn-impl - 0.11.0 | Thread terminated due
to uncaught exception: NeutronPortChangeListener-0',
'java.lang.IllegalArgumentException: Multiple entries with same key:
RouterIdsKey{_routerId=Uuid{_value=d2509c08-51eb-42c3-b80d-1da4abada791}}=RouterIds{_routerId=Uuid{_value=d2509c08-51eb-42c3-b80d-1da4abada791},
augmentation=[]} and
RouterIdsKey{_routerId=Uuid{_value=d2509c08-51eb-42c3-b80d-1da4abada791}}=RouterIds{getRouterId=Uuid{_value=d2509c08-51eb-42c3-b80d-1da4abada791},
augmentation=[]}. To index multiple values under a key, use
Multimaps.index.', '\tat
com.google.common.collect.Maps.uniqueIndex(Maps.java:1338)
~[bundleFile:?]', '\tat
com.google.common.collect.Maps.uniqueIndex(Maps.java:1293)
~[bundleFile:?]', '\tat
org.opendaylight.yangtools.yang.binding.CodeHelpers.compatMap(CodeHelpers.java:296)
~[bundleFile:?]', '\tat

Solution:
=========
Have added the check in vpnMaps DS update method call
before update the router details to the vpnMaps DS to
avoid the existing routerId information again.

JIRA: NETVIRT-1709

Signed-off-by: Karthikeyan Krishnan <karthikeyangceb007@gmail.com>
Change-Id: Ie795bcb2eaa30aa9ec71e5c7052c12b714865e1c