code: async function getTemplateFile(templateDir, stackName) {
    const f = await Promise.any(
      _.map(['.yml', '.yaml', '.json', ''], async(ext) =>
        await utils.fileExists(`${path.join(templateDir, stackName)}${ext}`)));
    if (f) {
      return f;
    }
    throw new Error(`Stack template "${stackName}" not found!`);
  }
docs: look for template having multiple possible file extensions