Comment 0 for bug 1757928

I met randomly UT failure within test_glance:
http://logs.openstack.org/43/554543/1/gate/openstack-tox-py27/6883f53/job-output.txt.gz

2018-03-21 10:39:37.989937 | ubuntu-xenial | ======================================================================
2018-03-21 10:39:37.990087 | ubuntu-xenial | FAIL: os_xenapi.tests.plugins.test_glance.GlanceTestCase.test_download_ok_extract_failed
2018-03-21 10:39:37.990132 | ubuntu-xenial | tags: worker-3
2018-03-21 10:39:37.990239 | ubuntu-xenial | ----------------------------------------------------------------------
2018-03-21 10:39:37.990301 | ubuntu-xenial | Traceback (most recent call last):
2018-03-21 10:39:37.990520 | ubuntu-xenial | File "/home/zuul/src/git.openstack.org/openstack/os-xenapi/.tox/py27/local/lib/python2.7/site-packages/mock/mock.py", line 1305, in patched
2018-03-21 10:39:37.990592 | ubuntu-xenial | return func(*args, **keywargs)
2018-03-21 10:39:37.990755 | ubuntu-xenial | File "os_xenapi/tests/plugins/test_glance.py", line 121, in test_download_ok_extract_failed
2018-03-21 10:39:37.990837 | ubuntu-xenial | mock_extract_tarball.assert_called_once()
2018-03-21 10:39:37.991063 | ubuntu-xenial | File "/home/zuul/src/git.openstack.org/openstack/os-xenapi/.tox/py27/local/lib/python2.7/site-packages/mock/mock.py", line 915, in assert_called_once
2018-03-21 10:39:37.991124 | ubuntu-xenial | raise AssertionError(msg)
2018-03-21 10:39:37.991248 | ubuntu-xenial | AssertionError: Expected 'extract_tarball' to have been called once. Called 0 times.
2018-03-21 10:39:37.991297 | ubuntu-xenial | Ran 269 tests in 1.263s
2018-03-21 10:39:37.991347 | ubuntu-xenial | FAILED (id=0, failures=1)