Fix `cqrs_dead_letters retry` command

[maxipavlovic]

Hey,

I've found that this Pull Request is breaking cqrs_dead_letters retry command.

_get_consumer_settings used to return 2 arguments and with this change it start to return tuple of 3,
which is breaking init_broker method in above mentioned command.
Exact line:

queue_name, dead_letter_queue_name = RabbitMQTransportService.get_consumer_settings()

is raising ValueError: too many values to unpack (expected 2)

Originally posted by @Szejdi in #39 (comment)