atomic-host: Stop writing /etc/hosts, use setup defaults
    
    The `setup` package has this same content, let's not duplicate it. The only
    difference between them today is trailing whitespace in our version.
    
    Just trying to reduce the amount of stuff we do here to avoid deltas with bare
    metal installs, containers, etc.
    
    Signed-off-by: Colin Walters <walters@verbum.org>