SD-405 : On test/development server redis and celery worker should be setup

Add the option to set CELERY_ALWAYS_EAGER through env variable
SD-335 : Archive ingest Back-end implementation

Added task_id on response
refactor(app): change the way how domain is registered

introduce `init_app` method in app modules which will get called when
a new app is being created via `get_app` factory
fix(domain): register resources defined via superdesk.domain
fix(celery): setup celery.Task only once
SD-373 : Archive ingest task result - back-end implementation

GET /archive_ingest/<task_id>
{'task_id': task_id, 'status': <PROGRESS/PENDING>, 'current': current,
'total': total}
Merge branch 'master' of https://github.com/ioanpocol/superdesk-server
SD-320 Scan S3 to populate content archive
SD-373 : Archive ingest task result - back-end implementation

Changed the celery task to use app as the current flask app.
SD-373 : Archive ingest task result - back-end implementation

Renamed a few functions
SD-320 Fix query to check if file already ingested into content archive
Merge remote-tracking branch 'ioan/master'
Merge remote-tracking branch 'adrian/sd-320'