Instead of always creating debug/ log directory, allow for multiple log groups selection

Previously when someone choose e.g INFO or WARNING log level,
both DEBUG and INFO/WARNING directories containing logs
(from that DEBUG and INFO/WARNING level and higher levels) were created.

Now its possible to manually select which groups will be created.
e.g:
- LOG_LEVELS = ["INFO"] - means that only info/ directory will be created
  and inside that directory there will be log files containing only INFO
  and higher level messages.
- LOG_LEVELS = ["DEBUG", "WARNING"] - means that debug/ and warning/
  directories will be created and debug/ will include all possible logs
  when warning/ directory will include logs starting from WARNING
  (so also ERROR level messages).