add firewall driver if we use quantum security group

fixes bug 1179820

we must set firewall driver if we want to use quantum security group because
quantum will disable security group if we do not change the default firewall
driver. Currently devstack will not change the default firewall driver
if we just running quantum server on a node which will cause nova unable to
security group information.

Change-Id: Ie274325decbf252630a237ed3d6ee3136eb259fe