Use assertGreater(len(x), 0) over assertTrue(len(x) > 0)

assertGreater provides a nicer error message if it fails.

Change-Id: I2c13d28c54ae3a9ff562448e7a16ab54d71b29b5
diff --git a/tempest/api/compute/servers/test_availability_zone.py b/tempest/api/compute/servers/test_availability_zone.py
index 76da317..00df86b 100644
--- a/tempest/api/compute/servers/test_availability_zone.py
+++ b/tempest/api/compute/servers/test_availability_zone.py
@@ -29,4 +29,4 @@
     def test_get_availability_zone_list_with_non_admin_user(self):
         # List of availability zone with non-administrator user
         availability_zone = self.client.list_availability_zones()
-        self.assertTrue(len(availability_zone['availabilityZoneInfo']) > 0)
+        self.assertGreater(len(availability_zone['availabilityZoneInfo']), 0)