Change assertTrue(isinstance()) by optimal assert

Some of tests use different method of assertTrue(isinstance(A, B))
The correct way is to use assertIsInstance(A, B) provided by testtools

Change-Id: Ie21fb409edf558409aa4bf5dfe59893c5b544aa8
diff --git a/manila_tempest_tests/tests/api/admin/test_export_locations.py b/manila_tempest_tests/tests/api/admin/test_export_locations.py
index d985d90..97db819 100644
--- a/manila_tempest_tests/tests/api/admin/test_export_locations.py
+++ b/manila_tempest_tests/tests/api/admin/test_export_locations.py
@@ -80,8 +80,8 @@
 
             # Check the format of ever-present summary keys
             self.assertTrue(uuidutils.is_uuid_like(export_location['id']))
-            self.assertTrue(isinstance(export_location['path'],
-                                       six.string_types))
+            self.assertIsInstance(export_location['path'],
+                                  six.string_types)
 
             if utils.is_microversion_ge(version, '2.14'):
                 self.assertIn(export_location['preferred'], (True, False))