Sorry, forgot to add. I could use Perl e.g.:

# config for server124
$config = YAML::Load('webserver.yamlt');
$config->{network}{ip_addresses} = [qw(10.0.0.124)];
$config->{software}{webserver} = 'apache';
$config->{features}{https} = 0;
return YAML::Dump($config);
[download]

But I'd fancy something that is more YAML-ish or template-language-ish, if such thing exists.