Allow checks to set result's log_url to specific artifact
    
    We want to allow checks to set log-urls in resultsdb to point onto a selected
    artifact file. Thus adding an `artifact` attribute to the `CheckDetail` class
    and then using the (possible) `artifact` variable as a storage for a relative
    path to a file in the `$artifactsdir`. `resutlsdb_directive` then checks for
    the presence of this variable in TAP, and replaces the result's `log_url`
    to the URL or the respective artifact file, if needed.
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D389