*: refactor iptables invocation
*: refactor iptables invocation

Centralise all invocations of iptables, so that the lock is used every
time it is called.

coreos/go-iptables,lyondhill/go-iptables,TerraTech/go-iptables,TerraTech/go-iptables,lyondhill/go-iptables,coreos/go-iptables