contrib/systemd: log_config.yaml: do not disable existing loggers

It turned out that merely configuring the root logger is not enough for
"catch-all" semantics. The logging subsystem also needs to be told not
to disable existing loggers (so that their messages will get propagated
to handlers up the logging hierarchy, not just silently discarded).

Signed-off-by: Ivan Shapovalov <intelfx100@gmail.com>