Give upload its own command
    
    Looks duplicate with new_sources, it makes it possible to do more for upload.
    At least, when use upload command to upload a sources file, if any exception is
    raised, fedpkg or rhpkg could give more specific message, like
    
        Could not execute upload
    
    instead of
    
        Could not execute new_sources
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>