Allow scheduling of update jobs for any arch (#57)
    
    Rather than hardcoding x86_64. I thought of ways to have the
    method schedule jobs for multiple arches in one call, but in
    the end didn't really like any of them, this should be much
    simpler. I don't think there'll be a significant performance
    difference either as ultimately there has to be one POST request
    per arch/flavor combination - we can't send a POST that creates
    jobs for more than one arch at a time.
    
    Fixes: #57