Return correct domains in cleanup service

Since 56317e0c99a9a, the full repsonse of all v3 IdentityClient
methods is returned. The cleanup service was just forgotten.

Change-Id: Ia2d3ef64a5057ef14cb2692f4a495dfa5ec18fff
Closes-Bug: #1490310
diff --git a/tempest/cmd/cleanup_service.py b/tempest/cmd/cleanup_service.py
index 365f37f..07d1d75 100644
--- a/tempest/cmd/cleanup_service.py
+++ b/tempest/cmd/cleanup_service.py
@@ -930,7 +930,7 @@
 
     def list(self):
         client = self.client
-        domains = client.list_domains()
+        domains = client.list_domains()['domains']
         if not self.is_save_state:
             domains = [domain for domain in domains if domain['id']
                        not in self.saved_state_json['domains'].keys()]