Explicitly set default_flow_style to False

In PyYAML 5.1 they changed this default in the library. Because
we weren't explicitly setting it, this broke our unit tests.
Since default_flow_style=False is usually what you want (hence why
PyYAML changed it), let's set it explicitly.

Also note that this was already being set for dump, but not for
dumps. This is also more consistent.

Change-Id: I3f7dfce5ceb5d3b802878c410406994cbb0988b6

oslo_serialization/tests/test_yamlutils.py

@@ -55,10 +55,12 @@ class BehaviorTestCase(base.BaseTestCase):
         ]
         dumped = yaml.dumps(payload)
         expected = textwrap.dedent('''\
-            - {foo: bar}
-            - {list: null}
-            - [one, two]
-            - {check: yaml, in: test}
+            - foo: bar
+            - list: null
+            - - one
+              - two
+            - check: yaml
+              in: test
         ''')
         self.assertEqual(dumped, expected)
 

oslo_serialization/yamlutils.py

@@ -67,7 +67,7 @@ def dumps(obj, is_safe=True):
             yaml_dumper = yaml.CSafeDumper
         else:
             yaml_dumper = yaml.SafeDumper
-    return yaml.dump(obj, Dumper=yaml_dumper)
+    return yaml.dump(obj, default_flow_style=False, Dumper=yaml_dumper)
 
 
 def dump(obj, fp, is_safe=True):