use plugin names from neutron_lib.plugins.constants

Switch plugin name references over to use the constants from
neutron_lib.plugins.constants in prep for
I5cbe779b401e5214f414ab1c732867fdd4d75651

Change-Id: Id96c8926483c504823c81a66a958d5941e52246d

neutron/plugins/common/constants.py

@@ -13,7 +13,6 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-from neutron_lib import constants
 from neutron_lib.plugins import constants as p_const
 
 
@@ -25,7 +24,7 @@ EXT_TO_SERVICE_MAPPING = {
     'fwaas': p_const.FIREWALL,
     'vpnaas': p_const.VPN,
     'metering': p_const.METERING,
-    'router': constants.L3,
+    'router': p_const.L3,
     'qos': p_const.QOS,
 }