Document security group when using ML2 plugin

Since the ML2 plugin can concurrently support different L2 agents (or
other mechanisms) with different configurations, Neutron developpers
recommend setting firewall_driver flag in ml2 configuration.

Change-Id: I7f97128a955ded99400e25a5ef9a990260df3bf7
Closes-bug: #1262678
backport: havana
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
(cherry picked from commit 4c5d9ba37bb2cebedb05aabab3452a5e0005f985)