Add in/out interface specification capability to rule define

Hi Alessandro,

  I've added two new parameters to rule define, in order to specify incoming interface. That way, you can specify rules like:

    -A POSTROUTING -t nat -o eth1 -j MASQUERADE

can be defined like

    iptables::rule { 'masquerade':
        table         => 'nat',
        chain         => 'POSTROUTING',
        target        => 'MASQUERADE',
        out_interface => 'eth1',
    }

Regards, Javier