adjust minimal log level at runtime

[localvar]

the minimal log level is currently statically configured via the configuration.
this means, when we meet an issue and need to change to the minimal log level to get more information from the log, we will have to shut down easegress and restart.
however, some issues cannot be reproduced after a restart.
so it would be a help if we could adjust the minimal log level at runtime.

[aniaan]

Consider adding LoggerController(SystemController), which listens to log events to change the log level , this relationship is like the relationship between RawConfigTrafficController and Pipeline

uber-go/zap#556

[xxx7xxxx]

Fixed by #1133