Implement OVN Ansible role.

Implements: blueprint ovn-controller-neutron-ansible

Depends-On: https://review.opendev.org/713422
Change-Id: Icd425dea85d58db49c838839d8f0b864b4a89a78

Change-Id: I799993728112a525e34cfbc4e786a10f0ed03be9

Add Neutron reference docs, especially a note around
using OVS native firewall driver on recent (4.3+) kernels [1].

[1]: https://docs.openstack.org/neutron/latest/admin/config-ovsfwdriver.html

Change-Id: I6994e364c116234b46f5d5e9f0a4666b83f86375
Closes-Bug: #1653987