Set up host firewall on VMs running a COS image (#1486)

By default, the Container-Optimized OS (COS) host firewall allows only
outgoing connections and incoming SSH connections. To allow other incoming
connections, we must open the ports.

This commit adds iptables rules on the netserver host to accept
connections from the netperf clients.

Signed-off-by: Saied Kazemi <saied@google.com>