Docker CE has added a default DROP policy to the FORWARD chain.  When
nova-compute runs on the controller, kolla ansible sets the
net.bridge.bridge-nf-call-iptables sysctl to 1, which causes iptables to
process frames forwarded across bridges.

Currently, the kayobe-overcloud-centos job is failing quite frequently
with timeouts when deploying bare metal compute. Experimentation with
iptables hasn't revealed why this only happens sometimes, or exactly
what traffic is being blocked, but opening up the firewall does seem to
fix the issue. We won't see this in production since control and compute
services are on separate hosts.

This change updates the iptables configuration used in CI to forward all
frames on the bridge, breth1.

Change-Id: If96437b73b9b5c58600ba1b004f53ee0c1f14398
Story: 2006534
Task: 36590