Unbreak test_created_router_interface (heat)

The test become broken after 49c0fe5630d0eb8dbc95b2df8f147fa3a33830ca.
This change adds the missing variable.

The test failed with:
AttributeError: 'NeutronResourcesTestJSON' object has no attribute
'ports_client'

Change-Id: Ib5b8391ee86ce0797f2935e75c2a7e92200ee3d1
diff --git a/tempest/api/orchestration/stacks/test_neutron_resources.py b/tempest/api/orchestration/stacks/test_neutron_resources.py
index e2ac455..a614c76 100644
--- a/tempest/api/orchestration/stacks/test_neutron_resources.py
+++ b/tempest/api/orchestration/stacks/test_neutron_resources.py
@@ -45,6 +45,7 @@
         super(NeutronResourcesTestJSON, cls).setup_clients()
         cls.network_client = cls.os.network_client
         cls.subnets_client = cls.os.subnets_client
+        cls.ports_client = cls.os.ports_client
 
     @classmethod
     def resource_setup(cls):