Fix test_cloud_config for PyYAML 5.1

PyYAML 5.1 changed default_flow_style=False. It's
backward incompatible.

Change-Id: I7b938b52b35d5546a674da50693c9c566f42120b
Story: #2005401
Task: #30395

heat/tests/openstack/heat/test_cloud_config.py

@@ -12,6 +12,7 @@
 #    under the License.
 
 import uuid
+import yaml
 
 import mock
 
@@ -53,6 +54,7 @@ class CloudConfigTest(common.HeatTestCase):
         kwargs = self.rpc_client.create_software_config.call_args[1]
         self.assertEqual({
             'name': self.config.physical_resource_name(),
-            'config': '#cloud-config\n{foo: bar}\n',
+            'config': '\n'.join(['#cloud-config',
+                                 yaml.safe_dump({'foo': 'bar'})]),
             'group': 'Heat::Ungrouped'
         }, kwargs)