Submit builds from stream branch
    
    This patch introduces the ability to submit multiple builds to Koji from
    stream branch based on configured build targets in local package config
    file. This applies to `fedpkg build` and chain build and scratch build
    still work as normal.
    
    The config file is optional. Without the config, packages could build
    from stream branch with global option --release and `fedpkg build` works
    as normal if there is no config file. If config file is created and
    build targets is set properly, fedpkg is able to read those targets and
    submit corresponding builds to Koji at once.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>