Improve Kafka consumer toggle

[Glutexo]

The Kafka message queue consumer is currently launched only if the configuration environment variables are set. These variables have default values though, which are useless because of this logic.

Improve this at least by either completely removing the condition or by removing the default values. It would be also possible to add a new toggle boolean variable to turn the consumer on/off. The best approach though would be to have a completely separate worker process with the consumer. Those could be then controlled simply by a Procfile.