atomic.conf: make a few pre-prod changes
    
    During the life of 2 week f26 atomic a few things will be set
    up differently than they are right now. Let's account for them:
    
    - Use dev yum repo for now
    - Don't use 'during compose' ostree repo location
    - Pull from non-updates ostree ref
    
    We'll switch all of these back to their previous values shortly
    after f26 is released and the other processes are in place.
    
    Signed-off-by: Dusty Mabe <dusty@dustymabe.com>