Install iptables-persistent package

Installs the iptables-persistent package in order to make iptable changes persistent across reboots.

This is necessary to allow native charms to communicate with Juju after a reboot.

Change-Id: Iacda7d36fa714991356c27a9858d4eccd77e4383
Signed-off-by: Adam Israel <adam.israel@canonical.com>