Define container parameters for logging levels

At the moment to define the log level for an handler an user have to specify the numeric value:

monolog.handler.rotating_file_error:
        class: Monolog\Handler\RotatingFileHandler
        arguments:
            - 'public://logs/error.log'
            - 10
            - '400'

It would be more useful if we define some container parameters for those values:

monolog.handler.rotating_file_error:
        class: Monolog\Handler\RotatingFileHandler
        arguments:
            - 'public://logs/error.log'
            - 10
            - '%monolog.level.error%'