lib/plugins/aws/provider/awsProvider.js/AwsProvider/getServerlessDeploymentBucketName
class AwsProvider:  getServerlessDeploymentBucketName(stage, region) {
    if (this.serverless.service.provider.deploymentBucket) {
      return BbPromise.resolve(this.serverless.service.provider.deploymentBucket);
    }
    const stackName = `${this.serverless.service.service}-${stage}`;
    return this.request('CloudFormation',
      'describeStackResource',
      {
        StackName: stackName,
        LogicalResourceId: 'ServerlessDeploymentBucket',
      },
      stage,
      region
    ).then((result) => result.StackResourceDetail.PhysicalResourceId);
  }