Add option '--log-level' to control log level for the top level log file

Add option `--log-level` to run command, to allow control the log level (debug, error, info, etc.) of the top level log file `logs/latest/debug.log`. Default to debug.

Note that the logs for the proper tests are not controlled by this flag, they will always contain all log messages.

Example: we are interested in error level conditions, `./run -t qemu --log-level=error` and then
following (tail -f) `logs/latest/debug/log` to watch if something wrong happens, otherwise the log will be empty.

Related to issue #179.