debian: skip iptables configuration

It seems that the debian/ubuntu firewall configuration is
more xenserver-core friendly, so no need to play with it.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
iptables: don't enable forwarding

This was only necessary as a workaround for the bridge, which
would by default filter all guest traffic via iptables (unlike
the OVS). Since we now configure the bridge not to do that
(via /etc/modprobe.d/bridge.conf) we nolonger need to play with
iptables forwarding rules.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #20 from djs55/iptables

Fix the firewall handling for debian/ubuntu