lib/utils/getServerlessConfigFile.js/getConfigFilePath
const getConfigFilePath = (servicePath, options = {}) => {
  if (options.config) {
    const customPath = path.join(servicePath, options.config);
    return fileExists(customPath).then(exists => {
      return exists ? customPath : null;
    });
  }

  const jsonPath = path.join(servicePath, 'serverless.json');
  const ymlPath = path.join(servicePath, 'serverless.yml');
  const yamlPath = path.join(servicePath, 'serverless.yaml');
  const jsPath = path.join(servicePath, 'serverless.js');

  return BbPromise.props({
    json: fileExists(jsonPath),
    yml: fileExists(ymlPath),
    yaml: fileExists(yamlPath),
    js: fileExists(jsPath),
  }).then(exists => {
    if (exists.yaml) {
      return yamlPath;
    } else if (exists.yml) {
      return ymlPath;
    } else if (exists.json) {
      return jsonPath;
    } else if (exists.js) {
      return jsPath;
    }

    return null;
  });
};