lib/plugins/aws/provider/awsProvider.js/AwsProvider/getRegion
class AwsProvider:  getRegion() {
    const defaultRegion = 'us-east-1';

    return _.get(this, 'options.region')
      || _.get(this, 'serverless.config.region')
      || _.get(this, 'serverless.service.provider.region')
      || defaultRegion;
  }