Handling SNAT cluster reboot failure

Handling SNAT cluster reboot failure

Description:
During Cluster reboot, the ext-router listener got the add event
following by update event(update of other parameters other then
enable/disable SNAT). When add event is recevied, a DPN is tried to
elect as NAPT switch, but since the CSS are not yet connect, Napt Switch
was not elected.

During update event, the Napt Switch was just elected without any
installation of flow(as there is no changes in enable/disable snat)
rersulting in failure.

Changes are done to handle update() event properly to install SNAT
related flows also when Napt Switch is elected.

Change-Id: I64f47dddf077ce7818ae6c76a497419dedb975f1
Signed-off-by: Chetan Arakere Gowdru <chetan.arakere@altencalsoftlabs.com>