log: make log.cfg{modules=...} work as box.cfg{log_modules=...}

Configuring log modules work differently with log.cfg and box.cfg:
box.cfg{log_modules=...} overwrites the current config completely while
log.cfg{modules=...} overwrites the currently config only for the
specified modules. Let's fix this inconsistency by making log.cfg behave
exactly as box.cfg.

Closes #7962

NO_DOC=bug fix