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. 

Commenting out this line https://github.com/fluent/fluentd/blob/50a90ac1cdc92860411be52280d5e1d0ec8bbbdb/lib/fluent/supervisor.rb#L108 suppressed this issue. 

This patch fixes the problem without removing the system config feature.  
Why this line is in ensure?