This is supported in kolla-ansible via the ironic_inspector_pxe_filter
variable, which can be added to globals.yml. The default value for that
variable changed in the Stein release from 'iptables' to 'dnsmasq',
since the iptables filter does not work with Docker CE [1].

This change removes the inspector_manage_firewall variable.

This change also adds an iptables rule in CI tests to allow DHCP packets
to be forwarded, to ensure bare metal servers can be deployed.

[1] https://bugs.launchpad.net/kolla-ansible/+bug/1823044

Depends-On: https://review.openstack.org/649673
Change-Id: Idac6777b4d97fbd17698fc2086ceb068d7b2e326
Related-Bug: #1823044