Make tempest L3 capable plugin aware.

With this patch, the public network config in tempest.conf will be
done for the plugins that support L3.

Change-Id: I820fe300fac45ff92d1281ff0c43ebc137783210
diff --git a/lib/neutron b/lib/neutron
index 8b0656b..6eabef5 100644
--- a/lib/neutron
+++ b/lib/neutron
@@ -115,6 +115,13 @@
 # nova vif driver that all plugins should use
 NOVA_VIF_DRIVER=${NOVA_VIF_DRIVER:-"nova.virt.libvirt.vif.LibvirtGenericVIFDriver"}
 
+# The next two variables are configured by plugin
+# e.g.  _configure_neutron_l3_agent or lib/neutron_plugins/*
+#
+# The plugin supports L3.
+Q_L3_ENABLED=${Q_L3_ENABLED:-False}
+# L3 routers exist per tenant
+Q_L3_ROUTER_PER_TENANT=${Q_L3_ROUTER_PER_TENANT:-False}
 
 # List of config file names in addition to the main plugin config file
 # See _configure_neutron_common() for details about setting it up