Rake script should not be overriding configuration

[csingh]

This line in the deimos:start task overrides the config.producers.backend value.

https://github.com/flipp-oss/deimos/blob/master/lib/tasks/deimos.rake#L10

I think this is bad practice because someone would be expecting to set all their configs in their deimos initializer file, but then when they try to run the app, this rake script would disable async kafka and cause some potential headaches and confusion. :P

Note though, that changing this would break backwards compatibility.

[dorner]

This might be a good opportunity to change the default backend to kafka from kafka_async since it's much easier to deal with issues and errors.