PyYAML 5.1 breaks unit tests

[Ben Nemec]

PyYAML changed the default for default_flow_style in https://github.com/yaml/pyyaml/commit/507a464ce62c933bf667b2296a96ad45f0147873

Because we weren't explicitly setting this value for our version of dumps, this broke our unit tests. We should explicitly set this since it's the expected behavior anyway. Also, we were already setting it in the dump function.

Unit test failure:
oslo_serialization.tests.test_yamlutils.BehaviorTestCase.test_dumps
-------------------------------------------------------------------

Captured traceback:
~~~~~~~~~~~~~~~~~~~
    b'Traceback (most recent call last):'
    b' File "/opt/stack/oslo.serialization/oslo_serialization/tests/test_yamlutils.py", line 63, in test_dumps'
    b' self.assertEqual(dumped, expected)'
    b' File "/opt/stack/oslo.serialization/.tox/py37/lib/python3.7/site-packages/testtools/testcase.py", line 411, in assertEqual'
    b' self.assertThat(observed, matcher, message)'
    b' File "/opt/stack/oslo.serialization/.tox/py37/lib/python3.7/site-packages/testtools/testcase.py", line 498, in assertThat'
    b' raise mismatch_error'
    b'testtools.matchers._impl.MismatchError: !=:'
    b"reference = '''\\"
    b'- foo: bar'
    b'- list: null'
    b'- - one'
    b' - two'
    b'- check: yaml'
    b' in: test'
    b"'''"
    b"actual = '''\\"
    b'- {foo: bar}'
    b'- {list: null}'
    b'- [one, two]'
    b'- {check: yaml, in: test}'
    b"'''"
    b''
    b''

[Ben Nemec (bnemec)]

Fixed by https://review.opendev.org/#/c/656813/