Configuration of /proc/sys/net/bridge/bridge-nf-call-iptables should be optional

[marun]

The launch script running weave on kube currently assumes it can manipulate /proc/sys/net/bridge/bridge-nf-call-iptables:

https://github.com/weaveworks/weave/blob/master/prog/weave-kube/launch.sh#L12-#L18

For my use-case I'm deploying kube on docker-in-docker. Since docker is already ensuring that bridge-nf-call-iptables is set on the host, and I'd rather not have to avoid mounting /proc/sys/net rw into the containers, it would be nice to be able to be able to disable this behavior in the launch script via an env variable (e.g. SET_BRIDGE_NF_CALL_IPTABLES, defaulted to true).