Fix test failure if client.cfg defines a logging_conf_file.

When /etc/luigi/client.cfg had a logging_conf_file entry, like

    logging_conf_file = /etc/luigi/logging.ini

test_cmdline_logger would fail with something like

    AssertionError: [call(None)] != [call('/etc/luigi/logging.ini')]
Merge pull request #371 from miku/test_cmdline_logger

Fix test failure if client.cfg defines a logging_conf_file.