[ostree] Use unique work and log paths
    
    Each variant.arch combination can have multiple ostree repos configured,
    so we need to make sure the filesystem paths don't clash.
    
    The paths used now are:
    
        logs/<arch>/<variant>/ostree-<x>/
        work/ostree-<x>/config_repo
    
    The x stands for a number identifying the task. It has no relation to
    actual contents of the repo.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>