Revert "update constraint for oslo.serialization to new release 2.21.2"

The update to 2.21.1 breaks glance unit tests:

FAIL: glance.tests.unit.v1.test_registry_api.TestRegistryAPI.test_update_all_image_members
----------------------------------------------------------------------
Traceback (most recent call last):
  File "glance/tests/unit/v1/test_registry_api.py", line 1629, in test_update_all_image_members
    content_type='json')
  File "glance/tests/utils.py", line 564, in get_api_response_ext
    self.assertEqual(res.status_int, http_resp)
  File "/data/dmueller/src/Cloud/glance/.tox/venv/lib/python2.7/site-packages/testtools/testcase.py", line 411, in assertEqual
    self.assertThat(observed, matcher, message)
  File "/data/dmueller/src/Cloud/glance/.tox/venv/lib/python2.7/site-packages/testtools/testcase.py", line 498, in assertThat
    raise mismatch_error
testtools.matchers._impl.MismatchError: 200 != 204

Related-Bug: #1728368

This reverts commit 47206e21375ddd6a33c4b1e5d65d95976a73d70a.

Also, set tox_install_siblings=false temporarily due to a bug
in tox_siblings which is causing incorrect versions to be
installed.

Change-Id: Ic91acb78452ec5f73015b47bbb3effd0d6bda497

.zuul.d/cross-jobs.yaml

@@ -14,6 +14,9 @@
          Use the specified tox environments (``ALL`` selects all).
     vars:
       zuul_work_dir: "{{ (zuul.projects | selectattr('required') | selectattr('name', 'match', '^(?!openstack/requirements)') | list)[0].src_dir }}"
+      # NOTE(jeblair): temporarily disabled tox_siblings until the fix
+      # in https://review.openstack.org/514058 lands.
+      tox_install_siblings: false
     files:
       - upper-constraints.txt
       - .zuul.d/cross-jobs.yaml

upper-constraints.txt

@@ -278,7 +278,7 @@ stestr===1.1.0
 urwid===1.3.1;python_version=='3.4'
 urwid===1.3.1;python_version=='3.5'
 singledispatch===3.4.0.3;python_version=='2.7'
-oslo.serialization===2.21.2
+oslo.serialization===2.21.1
 warlock===1.2.0
 exabgp===4.0.2
 sphinxcontrib-httpdomain===1.5.0