NAT not working after Replay-Based-Upgrade

NAT not working after Replay-Based-Upgrade

Description: we noticed that the SNAT flows 26->46, 36->46
flows are missing when RBU is perfomed.

Root cause : The flows are actually installed by iterating over the
DPNs retrieved from router-dpn-list, but as this is an operational
DS and is empty initially while doing RBU, the flows installation code is not hit

Solution: As we already have the NAPT switch ID, install the flows
without interating over the router-dpn-list.

Issue: NETVIRT-1617

Change-Id: Ie3b027356f754830e1cfd37d6c2daffdbf9bfa94
Signed-off-by: Chetan Arakere Gowdru <chetan.arakere@altencalsoftlabs.com>