Fix misuse of assertTrue

Replace 'assertTrue' with 'assertEqual'.

Change-Id: I725775d2eeb9d050aa2384499a9a282b1c4e5239
Closes-Bug: #1750537
diff --git a/tempest/api/image/v2/test_images_metadefs_namespace_tags.py b/tempest/api/image/v2/test_images_metadefs_namespace_tags.py
index 69bebfe..482e808 100644
--- a/tempest/api/image/v2/test_images_metadefs_namespace_tags.py
+++ b/tempest/api/image/v2/test_images_metadefs_namespace_tags.py
@@ -49,7 +49,7 @@
         # List namespace tags
         body = self.namespace_tags_client.list_namespace_tags(
             namespace['namespace'])
-        self.assertTrue(3, len(body['tags']))
+        self.assertEqual(3, len(body['tags']))
         self.assertIn(body['tags'][0]['name'], self.tag_list)
         self.assertIn(body['tags'][1]['name'], self.tag_list)
         self.assertIn(body['tags'][2]['name'], self.tag_list)