Add test case for update volume backup

Update volume backup is a new api implemented in volume v3 (since 3.9).
This patch is to add test case for this new feature.

Including:

[1] Add v3 backups_client as a library
[2] Add update backup api to v3 backups_client
[3] Add unit tests for update backup api
[4] Add test case: test_update_backup
[5] Add release note

Change-Id: Id50fcdbc41e9e170f3d467789f80c28bac01d434
diff --git a/tempest/api/volume/test_volumes_backup.py b/tempest/api/volume/test_volumes_backup.py
index 4b4aeec..1f91db6 100644
--- a/tempest/api/volume/test_volumes_backup.py
+++ b/tempest/api/volume/test_volumes_backup.py
@@ -140,3 +140,39 @@
             restored_volume_id)['volume']
 
         self.assertEqual('true', restored_volume_info['bootable'])
+
+
+class VolumesBackupsV39Test(base.BaseVolumeTest):
+
+    _api_version = 3
+    min_microversion = '3.9'
+    max_microversion = 'latest'
+
+    @classmethod
+    def skip_checks(cls):
+        super(VolumesBackupsV39Test, cls).skip_checks()
+        if not CONF.volume_feature_enabled.backup:
+            raise cls.skipException("Cinder backup feature disabled")
+
+    @decorators.idempotent_id('9b374cbc-be5f-4d37-8848-7efb8a873dcc')
+    def test_update_backup(self):
+        # Create volume and backup
+        volume = self.create_volume()
+        backup = self.create_backup(volume_id=volume['id'])
+
+        # Update backup and assert response body for update_backup method
+        update_kwargs = {
+            'name': data_utils.rand_name(self.__class__.__name__ + '-Backup'),
+            'description': data_utils.rand_name("volume-backup-description")
+        }
+        update_backup = self.backups_client.update_backup(
+            backup['id'], **update_kwargs)['backup']
+        self.assertEqual(backup['id'], update_backup['id'])
+        self.assertEqual(update_kwargs['name'], update_backup['name'])
+        self.assertIn('links', update_backup)
+
+        # Assert response body for show_backup method
+        retrieved_backup = self.backups_client.show_backup(
+            backup['id'])['backup']
+        for key in update_kwargs:
+            self.assertEqual(update_kwargs[key], retrieved_backup[key])