[buildinstall] Improve logging
    
    This patch fixes how logs are stored if lorax is used as buildinstall
    method. The logs for each variant are in a separate file now. If failure
    is allowed, the global log will now show why it failed.
    
    A couple tests are added as well.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>