Mount iptables lock file in weave-kube
iptables uses the file `/run/xtables.lock` so that if changes are
under way, another instance of the iptables command can wait till it
is finished.
We need to mount the lock file from the host to the container in order
for this mechanism to work at all.
Note we rely on kubelet touching the file before running any pods -
see https://github.com/kubernetes/kubernetes/pull/47212
Bryan Boreham