generate user_data.json with knife, launch instance with it and magic!

ec2_client.rb

# Author:: Adam Jacob <adam@opscode.com>
# Author:: Joshua Timberman <joshua@opscode.com>
#
# Copyright 2009-2010, Opscode, Inc
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#
# Chef Client Config File: /etc/chef/client.rb
# Automatically grabs configuration from ohai ec2 metadata.
# 
# Also note: sets the node's name to the EC2 instance ID, recommended as
# hostnames, fqdn's and IPs can be recycled.

require 'ohai'
require 'json'

o = Ohai::System.new
o.all_plugins
chef_config = JSON.parse(o[:ec2][:userdata])
if chef_config.kind_of?(Array)
  chef_config = chef_config[o[:ec2][:ami_launch_index]]
end

log_level        :info
log_location     STDOUT
node_name        o[:ec2][:instance_id]
chef_server_url  chef_config["chef_server"]

unless File.exists?("/etc/chef/client.pem")
  File.open("/etc/chef/validation.pem", "w", 0600) do |f|
    f.print(chef_config["validation_key"])
  end
end

if chef_config.has_key?("attributes")
  File.open("/etc/chef/client-config.json", "w") do |f|
    f.print(JSON.pretty_generate(chef_config["attributes"]))
  end
  json_attribs "/etc/chef/client-config.json"
end

validation_key "/etc/chef/validation.pem"
validation_client_name chef_config["validation_client_name"]

Mixlib::Log::Formatter.show_time = true

user_data.json

{
    "validation_key": "-----BEGIN RSA PRIVATE KEY-----\nsomelongreallyawesomekey\n-----END RSA PRIVATE KEY-----\n",
    "attributes": {
      "run_list": [
        "role[base]"
      ]
    },
    "chef_server": "http://chef.example.com:4000",
    "validation_client_name": "chef-validator"
}