Testcase failed due to real ceph cluster

Description
===========
I cloned the cinder repository of victoria branch and ran
tox tests. Unfortunately, the testcase which is 'cinder.tests.unit.volume.
drivers.test_rbd.RBDTestCase.test__set_keyring_attributes_cinderlib'
failed. This testcase was affected by the real ceph cluster which
had already produced the file whose path is '/etc/ceph/ceph.client.
admin.keyring'. However, testcases should be isolated instead of
affecting by their execution environments.

Steps to reproduce
==================
1. Deploy a ceph cluster on a node and make sure /etc/ceph/ceph.client.
admin.keyring exits.
2. Clone the cinder repository of victoria branch and ran
command such as 'tox -e py3'

Expected result
===============
All testcases succeed.

Actual result
=============
The testcase which is 'cinder.tests.unit.volume.
drivers.test_rbd.RBDTestCase.test__set_keyring_attributes_cinderlib'
failed.

Logs & Configs
==============
==============================
Failed 1 tests - output below:
==============================

cinder.tests.unit.volume.drivers.test_rbd.RBDTestCase.test__set_keyring_attributes_cinderlib
--------------------------------------------------------------------------------------------

Captured traceback:
~~~~~~~~~~~~~~~~~~~
    Traceback (most recent call last):

      File "/root/gitlab/cinder/cinder/tests/unit/volume/drivers/test_rbd.py", line 1638, in test__set_keyring_attributes_cinderlib
    self.assertIsNone(self.driver.keyring_data)

      File "/root/gitlab/cinder/.tox/py3/lib/python3.6/site-packages/testtools/testcase.py", line 430, in assertIsNone
    self.assertThat(observed, matcher, message)

      File "/root/gitlab/cinder/.tox/py3/lib/python3.6/site-packages/testtools/testcase.py", line 502, in assertThat
    raise mismatch_error

    testtools.matchers._impl.MismatchError: is not:
reference = None
actual = '''\
[client.admin]
\tkey = AQAEFtRfl81/CRAAkC9xP07CKa6d/q+3tNErWQ==
\tcaps mds = "allow *"
\tcaps mgr = "allow *"
\tcaps mon = "allow *"
\tcaps osd = "allow *"
'''