Create AWS CloudFormation template file

* `aws-cf-chef-server.json` is based on the cf template provided by
  Amazon, which can be found here:
  https://s3.amazonaws.com/cloudformation-templates-us-east-1/chef-server-ubuntu-configuration.template

The the template expects four parameters: ChefServerCookbookVersion,
InstanceType, KeyName and SSHLocation.

The KeyName parameter is required, the others have sensible defaults.