Don't configure router in flat network

Due to the fix [1] of neutron-refactor, some flat network usages of devstack
installation start fale.

This fix enables ML2_L3_PLUGIN to be set to empty to solve the problem.

By default l3_router_plugin.L3RouterPlugin will be set to ML2_L3_PLUGIN,
and for neutron, in such of configuration, router (ASA some others) will be
set into supported_extension_aliases,
then devstack will create a router that we do not want in a flat network.

Before fix [1], we can disable q-l3 to aviod the issue.
But now we don't, and we need this fix to disable the whole L3 plugin.

[1] https://review.openstack.org/318145

Change-Id: I61a2142d5121e0af4cc6cdf50e6bceafaf791fb0

lib/neutron_plugins/ml2

@@ -38,7 +38,9 @@ Q_ML2_PLUGIN_VLAN_TYPE_OPTIONS=${Q_ML2_PLUGIN_VLAN_TYPE_OPTIONS:-}
 Q_ML2_PLUGIN_EXT_DRIVERS=${Q_ML2_PLUGIN_EXT_DRIVERS-port_security}
 
 # L3 Plugin to load for ML2
-ML2_L3_PLUGIN=${ML2_L3_PLUGIN:-neutron.services.l3_router.l3_router_plugin.L3RouterPlugin}
+# For some flat network environment, they not want to extend L3 plugin.
+# Make sure it is able to set empty to ML2_L3_PLUGIN.
+ML2_L3_PLUGIN=${ML2_L3_PLUGIN-neutron.services.l3_router.l3_router_plugin.L3RouterPlugin}
 
 function populate_ml2_config {
     CONF=$1