Improve logging configuration in celery's setup_log_handler function and logging of the listener

logging config

• allow to pass the loglevel as a string,
• kill useless tmp variables,
• add a filter to prevent amqp's heartbeat_tick debug messages.

listener

• do use a dedicated logger instead of the root logger,
• add a couple of logging statements (in perform_action methods),
• replace the --verbose cli option by --log-level

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Migrated from D885 (view on Phabricator)