Allow configuration by 'dist' instead of 'milestone'
    
    This is why the reports for Atomic lost their special config
    lately - I changed fedfind so the 'milestone' attr wasn't being
    kinda incorrectly used as an identifier of the type of compose
    sometimes, and instead left that job to the 'dist' attr. I
    forgot that check-compose relied on this, though. Since the
    'milestone' for two-week Atomic nightlies is now 'Nightly', not
    'Atomic', this broke. Let's rejig check-compose to use 'dist',
    then we just have to tweak the config in infra ansible and it'll
    work again.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>