no such option: policy_file error raised when running a certain UT

[Jeffrey Zhang]

When running a certain UT, for example
$ tox -e py27 -- --tests keystone.tests.test_keystoneclient
All the test is failed. And got following err msg.

======================================================================
ERROR: keystone.tests.test_keystoneclient.KcMasterTestCase.test_user_update_tenant_404
----------------------------------------------------------------------
_StringException: Traceback (most recent call last):
  File "/home/jeffrey/openstack/keystone/keystone/tests/test_keystoneclient.py", line 35, in setUp
    super(CompatTestCase, self).setUp()
  File "/home/jeffrey/openstack/keystone/keystone/tests/core.py", line 259, in setUp
    self.opt(policy_file=etcdir('policy.json'))
  File "/home/jeffrey/openstack/keystone/keystone/tests/core.py", line 296, in opt
    CONF.set_override(k, v)
  File "/home/jeffrey/openstack/keystone/.tox/py27/local/lib/python2.7/site-packages/oslo/config/cfg.py", line 1579, in __inner
    result = f(self, *args, **kwargs)
  File "/home/jeffrey/openstack/keystone/.tox/py27/local/lib/python2.7/site-packages/oslo/config/cfg.py", line 1825, in set_override
    opt_info = self._get_opt_info(name, group)
  File "/home/jeffrey/openstack/keystone/.tox/py27/local/lib/python2.7/site-packages/oslo/config/cfg.py", line 2095, in _get_opt_info
    raise NoSuchOptError(opt_name, group)
NoSuchOptError: no such option: policy_file

It seems related to this commit https://review.openstack.org/48111