celery: deprecated celeryd replacement

* As of the celery version 3.1, `celeryd` is deprecated in favour of
  `celery worker`. It will be completely removed with the 3.2 version.

Signed-off-by: Daniel Lovasko <daniel.lovasko@cern.ch>
legacy: fix import in bibstat cli

* Fixes import in bibstat script.  (closes #2293)

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>