Disable JSON config from command line

In HDK context, the HGRC file references a hg-logging.json configuration file. But the RSPec test setup need a simple way to disable it, so that the logs of the HGitaly server spawned for theses tests don't end up in the same place as when the application is running for good.

Unfortunately, we're not currently allowing logging.config_json to be the empty string: we strictly test for None instead, and that is not appropriate in this case.