lib/classes/Variables.js/Variables/getValueFromS3
class Variables:  getValueFromS3(variableString) {
    const groups = variableString.match(this.s3RefSyntax);
    const bucket = groups[1];
    const key = groups[2];
    return this.serverless
      .getProvider('aws')
      .request(
        'S3',
        'getObject',
        {
          Bucket: bucket,
          Key: key,
        },
        { useCache: true }
      ) // Use request cache
      .then(response => BbPromise.resolve(response.Body.toString()))
      .catch(err => {
        const errorMessage = `Error getting value for ${variableString}. ${err.message}`;
        return BbPromise.reject(new this.serverless.classes.Error(errorMessage));
      });
  }