only stop/disable simple service if it is installed
    
    The SimpleServiceInstance uninstaller assument that the service to
    uninstall was always present on the system. This may not be valid in
    some cases (e.g. containerized deployments) and thus we need to change
    the service state only when we know that the unit file exists.
    
    https://pagure.io/freeipa/issue/6977
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>