bridge-nf-call-iptables appears to only be relevant when the containers are
attached to a Linux bridge, which is usually the case with default Kubernetes
setups, docker, and flannel. That ensures that the container traffic is
actually subject to the iptables rules since it traverses a Linux bridge
and bridged traffic is only subject to iptables when bridge-nf-call-iptables=1.
But with other networking solutions (like openshift-sdn) that don't use Linux
bridges, bridge-nf-call-iptables may not be not relevant, because iptables is
invoked at other points not involving a Linux bridge.
The decision to set bridge-nf-call-iptables should be influenced by networking
plugins, so push the responsiblity out to them. If no network plugin is
specified, fall back to the existing bridge-nf-call-iptables=1 behavior.
