in.yaml

---
one::services::enabled: true
one::two::abc: 8.45
one::two::three: this is the string
puppet4::ca_server: otherhostname
puppet4::server: hostname

new.yaml

% ./make_pretty_yaml.rb test.yaml
---
one::two::three: this is the string
one::services::enabled: true
one::two::abc: 8.45

puppet4::ca_server: otherhostname
puppet4::server: hostname

pretty_yaml.rb

#!/opt/puppetlabs/puppet/bin/ruby

require "yaml"

data = YAML.load_file(ARGV.shift)

class_order = [String, TrueClass, FalseClass, Fixnum, Float, Array, Hash]

puts "---"

last_module = nil

data.sort_by do |k, v|
  [
    k.split("::").first,
    class_order.index(v.class)
  ]
end.each do |k, v|
  mod_name = k.split("::").first

  puts if last_module && mod_name != last_module

  last_module = mod_name

  puts YAML.dump(Hash[[[k, v]]]).split(/\n/)[1..-1].join("\n") + "\n"
end