Add note for NoopFirewallDriver

Complement description and Configuration When Nova let Neutron to
handle the firewall.
Add case descrition when setting "firewall_driver = neutron.agent
.firewall.NoopFirewallDriver" both in Nova and Neutron.

Change-Id: I80211afaf335c8e50c72a907d6b15e9b5cbf7dd8
Closes-Bug: #1232965
(cherry picked from commit 650984d9156a324efd4e08c2e8065ca4b5d24d31)