Support ruby Hash object in heat `template` option

[julienvey]

os.stacks = [{
  name: 'mystack',
  template: {
    heat_template_version: '2013-05-23',
    description: 'Simple template to deploy a single compute instance',
    resources: {
      my_instance: {
        type: 'OS::Nova::Server',
        properties: {
          key_name: 'julien-mac',
          image: 'CoreOS',
          flavor: '1_vCPU_RAM_512M_HD_10G',
      }
    }
  }
}]

For the implementation, a simple solution consists of converting this object in json with the .to_json instruction