Configure 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: I62d0d1b2aac88a11e5f3e80e98953c71cfc98629
Closes-bug: #1262678
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>