Replace Q_L3_ENABLED by is_service_enabled q-l3

This patch replaces Q_L3_ENABLED with is_service_enabled q-l3.
Both of them idicates wherever Neutron L3 agent is enabled or not.

Change-Id: I33f0f5a6174d1d170bc2ac1c2e3a096d88d17cc1
diff --git a/lib/tempest b/lib/tempest
index e556935..298d030 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -16,7 +16,6 @@
 #   - ``BASE_SQL_CONN`` ``lib/database`` declares
 #   - ``PUBLIC_NETWORK_NAME``
 #   - ``Q_ROUTER_NAME``
-#   - ``Q_L3_ENABLED``
 #   - ``VIRT_DRIVER``
 #   - ``LIBVIRT_TYPE``
 #   - ``KEYSTONE_SERVICE_PROTOCOL``, ``KEYSTONE_SERVICE_HOST`` from lib/keystone
@@ -239,7 +238,7 @@
 
     ssh_connect_method=${TEMPEST_SSH_CONNECT_METHOD:-$ssh_connect_method}
 
-    if [ "$Q_L3_ENABLED" = "True" ]; then
+    if is_service_enabled q-l3; then
         public_network_id=$(neutron net-list | grep $PUBLIC_NETWORK_NAME | \
             awk '{print $2}')
     fi