argparse: change --arch to be a single value instead of a string
    
    This makes it easier to handle this value in formula and pass it to
    other directives (like a bash directive), or modify it (append something
    to it).
    
    This is a substantial change. None of existing tasks seem to be making
    use of multi-valued ${arch}. Generic tasks usually use 'all' arch value
    in koji directive and process everything in one pass. Arch-specific
    tasks will mostly want to handle just a single arch in a single pass.
    That will also help us keep our sanity.
    
    This patch is helpful for resolving T894 (making depcheck/rpmdeplint a
    per-arch task).
    
    As a convenience, all directives now support both str and a list of str
    in `arch` parameters.
    
    Differential Revision: https://phab.qa.fedoraproject.org/D1171