Make a subcommand for creating a new compose
    
    This change would be flexible for odcs script to add more commands in
    the future. For example, to get a compose info, or delete a compose.
    
    This patch removes code that check any missing required arguments,
    source or source_type. Instead, argparse module will handle that.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>