Add os-iptables-stateful script for managing iptables

Applies iptables configuration based on an input file and the current
iptables state. This script is used to perform idempotent operations,
eg to prevent adding duplicate rules.

Sample input:

-N stunnel-INPUT
-A stunnel-INPUT -p tcp -m tcp --dport 4433 -j REJECT
-A stunnel-INPUT -j RETURN
-I INPUT -p tcp -j stunnel-INPUT

Unlike 'iptables-restore --noflush' the script is idempotent.

Change-Id: I8eef1361ea90647507cf596f7bfc81815e32a96e
Partially implements: blueprint os-iptables-scripts
Merge "Add os-iptables-stateful script for managing iptables"