[1479] Allow setting of worker name

Worker name should be unique.

"You can also start multiple workers on the same machine. If you do so be sure
to give a unique name to each individual worker by specifying a host name with
the --hostname|-n argument"

See: http://celery.readthedocs.org/en/2.4-archived/userguide/workers.html
Merge pull request #1 from datagovuk/1479-allow-different-worker-names

[1479] Allow setting of worker name