Provide better message when no IPv4 addresses found

Provide a better message when no IPv4 addresses are found. This should
make it more clear when the assertion fails.

Change-Id: Icd6fd034c7e969ea2fab59aa2bf9bebe94956537
diff --git a/tempest/scenario/manager.py b/tempest/scenario/manager.py
index a10dc83..7251baf 100644
--- a/tempest/scenario/manager.py
+++ b/tempest/scenario/manager.py
@@ -757,6 +757,8 @@
                     for fxip in p["fixed_ips"]
                     if netaddr.valid_ipv4(fxip["ip_address"])]
 
+        self.assertNotEqual(0, len(port_map),
+                            "No IPv4 addresses found in: %s" % ports)
         self.assertEqual(len(port_map), 1,
                          "Found multiple IPv4 addresses: %s. "
                          "Unable to determine which port to target."