add Release.wait() to wait for the compose to complete
    
    I'm not 100% super-convinced that this really belongs here, but
    jskladan said it should be in fedfind not openqa_trigger, so
    fine, whatever. Here it is. He suggested making koji_done and
    pungi_done into methods with 'wait' capability, but I didn't
    want to change the interface, so add a new method instead,
    which waits for them. Has some configurability for timeouts and
    retries.