Use logging module to log information about the loop devices used by the test harness

This needs:

* pass a ```--log-level``` option to the ```test_harness.py``` script
* use the value of the ```--log-level``` option to configure the logger
* add the logging at the info level to the ```_make_loopbacked_devices``` method or possibly the ```_run_command``` method

Related: https://github.com/stratis-storage/stratisd/pull/3452