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/tempest b/lib/tempest
index 803b740..7932fe6 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -15,6 +15,7 @@
 #   - ``PUBLIC_NETWORK_NAME``
 #   - ``Q_USE_NAMESPACE``
 #   - ``Q_ROUTER_NAME``
+#   - ``Q_L3_ENABLED``
 #   - ``VIRT_DRIVER``
 #   - ``LIBVIRT_TYPE``
 #   - ``KEYSTONE_SERVICE_PROTOCOL``, ``KEYSTONE_SERVICE_HOST`` from lib/keystone
@@ -200,7 +201,7 @@
 
     ssh_connect_method=${TEMPEST_SSH_CONNECT_METHOD:-$ssh_connect_method}
 
-    if is_service_enabled q-l3; then
+    if [ "$Q_L3_ENABLED" = "True" ]; then
         public_network_id=$(neutron net-list | grep $PUBLIC_NETWORK_NAME | \
             awk '{print $2}')
         if [ "$Q_USE_NAMESPACE" == "False" ]; then