lib/classes/Variables.js/Variables/getValueFromSsm
class Variables:  getValueFromSsm(variableString) {
    const groups = variableString.match(this.ssmRefSyntax);
    const param = groups[1];
    const decrypt = (groups[2] === 'true');
    return this.serverless.getProvider('aws').request(
      'SSM',
      'getParameter',
      {
        Name: param,
        WithDecryption: decrypt,
      },
      { useCache: true }) // Use request cache
      .then(response => BbPromise.resolve(response.Parameter.Value))
      .catch((err) => {
        if (err.statusCode !== 400) {
          return BbPromise.reject(new this.serverless.classes.Error(err.message));
        }

        return BbPromise.resolve(undefined);
      });
  }