util: data: Use the correct columns dictionary
    
    The .columns property was never defined to be subscriptable,
    and this has apparantly been dropped in the recent sqlalchemy
    release.
    This patch switches it to use the documented .c, which gives
    access to the same information, but is subscriptable.
    
    Fixes: #212
    Signed-off-by: Patrick Uiterwijk <puiterwijk@redhat.com>
    Reviewed-by: Rob Crittenden <rcritten@redhat.com>