fix flaky test on tests::image::test_utils.py::ReleaseUtilsTest::test_get_docker_client

[lonly7star]

This PR aims to fix the flaky test on tests::image::test_utils.py::ReleaseUtilsTest::test_get_docker_client so the test could pass single run, multiple test run, and random test run.

The result

The test will fail on the multipe re-runs with pytest flake-finder, the following error will raised start on the second test run
> client_mock.assert_called_with( base_url=None, tls=None, version=DEFAULT_DOCKER_API_VERSION)
E AssertionError: expected call not found.
E Expected: mock(base_url=None, tls=None, version='1.21')
E Actual: mock(base_url='http://127.0.0.1', version='1.40', tls=<Mock name='mock.tls.TLSConfig()' id='140319742926704'>)

Steps to reproduce the issue

1. install pytest flaky finder with pip install pytest-flakefinder
2. run pytest with flake-finder with command pytest -k tests/image/test_utils.py --flake-finder

Issue of the code

The reason is that the test assumes the os.environ variables DOCKER_HOST, DOCKER_TLS_VERIFY, and DOCKER_API_VERSION are None at the beginning of the testing method. However, those variables will be initialized in the later part of the test and didn't get removed. Therefore started on the second test run the test will fail due to different code flow on `utils.opy::get_docker_client'

Proposed solution

Adding three if statement to delete those environmental variables if they are not None, so the test function stay fresh in each run.