lib/neutron: Fix some settings
lib/neutron: Fix some settings

* Set ml2_type_flat setting so that the public flat network is created
correctly

* Set securitygroup driver correctly

It should be set as:

[securitygroup]
firewall_driver = iptables

Change-Id: I7369b45fbc5a47ce958693c67a1902a8cb24f367

richliu/devstack,nawawi/openstack,richliu/devstack,group-policy/devstack,bswartz/devstack,noironetworks/devstack,group-policy/devstack,bswartz/devstack,noironetworks/devstack,dtroyer/devstack,openstack-dev/devstack,dtroyer/devstack,openstack-dev/devstack,nawawi/openstack