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
fix(amazon): fix how config is populated from env

so that it can be set by other means
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