Add lost function of is_provider_network

This is a follow up patch of [1].
In [1], source has been moved from lib/neutron-legacy to lib/neutron_plugins/services/l3.
However, one necessary function of is_provider_network is lost.
And this cause devstack install fail.

[1]https://review.openstack.org/168438/

Change-Id: I413b3577ec5b11ee0ee01f2368364117962494bb
diff --git a/lib/neutron_plugins/services/l3 b/lib/neutron_plugins/services/l3
index 4c4730e..5ff8ea5 100644
--- a/lib/neutron_plugins/services/l3
+++ b/lib/neutron_plugins/services/l3
@@ -360,3 +360,10 @@
         _neutron_set_router_id
     fi
 }
+
+function is_provider_network {
+    if [ "$Q_USE_PROVIDER_NETWORKING" == "True" ] && [ "$Q_L3_ENABLED" == "False" ]; then
+        return 0
+    fi
+    return 1
+}