GET backup before asserting 'volume_id' and 'snapshot_id'

Recently, commit c96757b275a77a745c6c80947bdd0bcb2de7eefd was merged
to remove redundant cleanups in volume backup tests. But there was a
bug in the change that assumed 'volume_id' and 'snapshot_id' are
returned from the backup create API when they are not.

We need to GET the backup in the API in order to obtain the 'volume_id'
and 'snapshot_id' for the asserts.

Closes-Bug: #1796207

Change-Id: I640fe41ec6049bfd980dfcbb6cae95a9a3e97b66
diff --git a/cinder_tempest_plugin/api/volume/test_volume_backup.py b/cinder_tempest_plugin/api/volume/test_volume_backup.py
index 8c81543..3547d71 100644
--- a/cinder_tempest_plugin/api/volume/test_volume_backup.py
+++ b/cinder_tempest_plugin/api/volume/test_volume_backup.py
@@ -41,6 +41,9 @@
         backup = self.create_backup(
             volume_id=volume['id'],
             snapshot_id=snapshot['id'])
+        # Get a given backup. We need to do this to get the volume_id and
+        # snapshot_id of the backup. They are not returned by the create API.
+        backup = self.backups_client.show_backup(backup['id'])['backup']
         self.assertEqual(volume['id'], backup['volume_id'])
         self.assertEqual(snapshot['id'], backup['snapshot_id'])