Update Neutron reference docs

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