ipatests: do not collect systemd journal when logfile_dir is missing
    
    If logs aren't collected to logfile_dir, skip collection of systemd
    journal.
    
    Related https://pagure.io/freeipa/issue/6971
    
    Signed-off-by: Tomas Krizek <tkrizek@redhat.com>
    Reviewed-By: Martin Basti <mbasti@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>