Use assertGreater(Equal) over assertTrue

As in I2c13d28c54ae3a9ff562448e7a16ab54d71b29b5, assertGreater/
assertGreaterEqual provides a nicer error message when failing if
no additional error message is specified.
This is to fix other occurrences of this kind.

Change-Id: I17a55f37026e26355a214541776975ad2efe70b2
diff --git a/tempest/api/compute/servers/test_server_addresses.py b/tempest/api/compute/servers/test_server_addresses.py
index 864f38f..d31b6f8 100644
--- a/tempest/api/compute/servers/test_server_addresses.py
+++ b/tempest/api/compute/servers/test_server_addresses.py
@@ -49,9 +49,9 @@
 
         # We do not know the exact network configuration, but an instance
         # should at least have a single public or private address
-        self.assertTrue(len(addresses) >= 1)
+        self.assertGreaterEqual(len(addresses), 1)
         for network_name, network_addresses in six.iteritems(addresses):
-            self.assertTrue(len(network_addresses) >= 1)
+            self.assertGreaterEqual(len(network_addresses), 1)
             for address in network_addresses:
                 self.assertTrue(address['addr'])
                 self.assertTrue(address['version'])