initialize FAS only when needed
    
    It's good to avoid unnecessary code in `__init__.py`. FAS can be loaded only
    when it's needed. Thanks to this, developers can finally avoid specifying
    `DEV=true` before every command (if they modify their `settings.py` accordingly),
    because FAS instance is now loaded based on config values and not envvars.
    
    Merges: https://pagure.io/fedora-qa/blockerbugs/pull-request/170