checks.py: show warning message for alias option
    
    Show warning message for any alias option find in config instance.
    Example warning message:
    
    WARNING: Config option 'product_name' is deprecated and now an alias to
    'release_name', please use 'release_name' instead. In:
    {'release_name': 'dummy product', 'product_name': 'dummy product'}
    
    Signed-off-by: Qixiang Wan <qwan@redhat.com>