Fix system config using double memory

The current code calls `read_config` twice on the parent (supervisor) process and the child (main) process. 
Because the member variable `@config_data` is overwritten in the child process, the memory usage for `@config_data` becomes double. This causes a problem on an environment whose conf file is large like 10,000 lines. 

This patch fixes the problem. 

cf. https://gist.github.com/sonots/2a146a8a54e76be83d7d
Removed this trick. Now, `read_config` and `apply_system_config` uses the default (or commad line option arguments) log_level. After the system_config is applied, the log_level is changed.