backend: run playbooks with timeout
    
    In some circumstances, playbook spawning can indefinitely stuck on something
    and therefore block the builder VM without a possibility to free it.
    In such case, a manual intervention is required, the builder needs to be
    killed and then copr-backend can spawn a new one.
    
    When timeouting the `ansible-playbook' command, we can ultimately kill
    the spawning process, making a useless builder instance. However, we
    automatize killing such builders via Cron task, so we eventually free
    it and get a new one.
    
    Merges: #1091