config.yaml

- ['foo', 'bar']
- ['a', 'b']

gen-hosts.rb

#!/usr/bin/env ruby

require 'yaml'
require 'erb'

hosts = YAML.load(open(ARGV[0]))

template = ERB.new <<-EOF

  127.0.0.1 localhost

  x.x.x.x puppet

  <% hosts.each do |hostname, domain| %>
    <%= hostname %> <%= hostname %>.<%= domain %>
  <% end %>

EOF

puts template.result(binding)