Sporadic failures of TestDiscovery.test_get_config_file

[Ilya Shakhat]

Tempest test case tempest.tests.cmd.test_verify_tempest_config.TestDiscovery.test_get_config_file
fails randomly with:

Traceback (most recent call last):
  File "/home/zuul/src/git.openstack.org/openstack/tempest/tempest/tests/cmd/test_verify_tempest_config.py", line 591, in test_get_config_file
    os.path.isfile('%s/tempest.conf.sample' % (conf_dir)))
  File "/home/zuul/src/git.openstack.org/openstack/tempest/.tox/py35/lib/python3.5/site-packages/unittest2/case.py", line 702, in assertTrue
    raise self.failureException(msg)
AssertionError: False is not true

The issue affects stability of 'openstack-tox-pyXX' jobs (though on py3x is occurs more frequently comparing to py27).

Recent failed jobs:
http://logs.openstack.org/09/608609/3/check/openstack-tox-py35/745b7b9/testr_results.html.gz
http://logs.openstack.org/35/523935/7/check/openstack-tox-py36/19d4973/testr_results.html.gz

Root cause
----------

The test case asks oslo.config to generate a configuration file and then immediately verifies file existance:
(http://git.openstack.org/cgit/openstack/tempest/tree/tempest/tests/cmd/test_verify_tempest_config.py#n596)

    init_cmd.generate_sample_config(os.path.join(conf_dir, os.pardir))
    self.assertTrue(os.path.isfile(local_sample_conf_file),
                    local_sample_conf_file)

However oslo_config.generate() function does not close the file, as result the data stays in memory and not always written to file system immediately.

Fix
---

The fix is already merged into oslo.config repo https://review.openstack.org/#/c/609011

[Ilya Shakhat (shakhat)]

Dup of https://bugs.launchpad.net/tempest/+bug/1796073