[docker-orchagent]: Increase ndppd kernel poll interval (#7456)

Why I did it
ndppd by default reads /proc/net/ipv6_route ever 30 seconds. Since T1s advertise so many routes to ToRs, this file is extremely large, and reading it causes ndppd's CPU usage to spike every 30 seconds

How I did it
Increase the delay for reading this file to the maximum possible value (max integer value), which will result in CPU spikes every ~24 days instead of every 30 seconds

How to verify it
Start ndppd with the new config file, confirm that no CPU spikes are seen except at startup

Signed-off-by: Lawrence Lee <lawlee@microsoft.com>

dockers/docker-orchagent/ndppd.conf.j2

@@ -21,7 +21,7 @@
 {%              set _x = proxy_interfaces[intf].append(prefix) %}
 {%          endif %}
 {%      endfor -%}
-
+route-ttl 2147483647
 {%      for intf, prefix_list in proxy_interfaces.items() %}
 {%          if prefix_list %}
 

src/sonic-config-engine/tests/sample_output/py2/ndppd.conf

@@ -4,6 +4,7 @@
 #
 # Config file for ndppd, the NDP Proxy Daemon
 # See man page for ndppd.conf.5 for descriptions of all available options
+route-ttl 2147483647
 
 proxy Vlan1000 {
     rule fc02:1000::/64 {

src/sonic-config-engine/tests/sample_output/py3/ndppd.conf

@@ -4,6 +4,7 @@
 #
 # Config file for ndppd, the NDP Proxy Daemon
 # See man page for ndppd.conf.5 for descriptions of all available options
+route-ttl 2147483647
 
 proxy Vlan1000 {
     rule fc01:1000::/64 {