get_workers returns queues, not workers

More confusion between workers and queues (see bliss/blissoda!177 (comment 381761)).

This was my fault from the beginning: !136 (comment 306559). I thought the concept of queue's would be too confusing for the client but it has caused more harm than good. Also in the Ansible playbooks this created troubles.

We'll need to deprecate the ewoksserver API as well: ewoksserver!100 (merged).