Add an iptable rule to allow icmp for antrea-gw0 for NodeLatencyMonitor feature

[luolanzone]

Describe the problem/challenge you have

When I tried the NodeLatencyMonitor feature in a local testbed where the ICMP query is blocked by default, the feature will not work as expected because it relys on ICMP request to get the latency between Nodes.

Describe the solution you'd like

When I added an iptable rule 'iptables -A INPUT -p icmp --icmp-type echo-request -j ACCEPT' on the Node, the feature is back to normal.
We can add a new rule to limit it to antrea-gw0 only with iptables -A INPUT -i antrea-gw0 -p icmp --icmp-type echo-request -j ACCEPT from Antrea side to make the feature work when the Node ICMP is blocked by default.