Add failure ports to error message

On the gate, test_dualnet_multi_prefix_slaac fails due to duplicated
ports on the test. However, the error message doesn't contain actual
ports information. This patch adds it for debugging.

Change-Id: I198ae97b954c01581a037923494949527fed8efa
Related-Bug: #1505478
diff --git a/tempest/scenario/test_network_v6.py b/tempest/scenario/test_network_v6.py
index a9394cb..3df92cf 100644
--- a/tempest/scenario/test_network_v6.py
+++ b/tempest/scenario/test_network_v6.py
@@ -143,8 +143,9 @@
                  self._list_ports(device_id=sid,
                                   network_id=self.network_v6.id)]
         self.assertEqual(1, len(ports),
-                         message="Multiple IPv6 ports found on network %s"
-                         % self.network_v6)
+                         message=("Multiple IPv6 ports found on network %s. "
+                                  "ports: %s")
+                         % (self.network_v6, ports))
         mac6 = ports[0]
         ssh.turn_nic_on(ssh.get_nic_name(mac6))