Fix L3 plugin to match with upstream

An upstream code was changed to get the tenant_id for router.
This patch fixes L3 plugin to match with the upstream fix

Change-Id: I20ba82d014cbe8fabae59cfeded53dd1f8af88f2

networking_arista/l3Plugin/l3_arista.py

@@ -100,7 +100,7 @@ class AristaL3ServicePlugin(db_base_plugin_v2.NeutronDbPluginV2,
     def create_router(self, context, router):
         """Create a new router entry in DB, and create it Arista HW."""
 
-        tenant_id = self._get_tenant_id_for_create(context, router['router'])
+        tenant_id = router['router']['tenant_id']
 
         # Add router to the DB
         with context.session.begin(subtransactions=True):