lib/classes/Variables.js/Variables/getValueFromCf
class Variables:  getValueFromCf(variableString) {
    const regionSuffix = variableString.split(':')[0].split('.')[1];
    const variableStringWithoutSource = variableString.split(':')[1].split('.');
    const stackName = variableStringWithoutSource[0];
    const outputLogicalId = variableStringWithoutSource[1];
    const options = { useCache: true };
    if (!_.isUndefined(regionSuffix)) {
      options.region = regionSuffix;
    }
    return this.serverless
      .getProvider('aws')
      .request('CloudFormation', 'describeStacks', { StackName: stackName }, options)
      .then(result => {
        const outputs = result.Stacks[0].Outputs;
        const output = outputs.find(x => x.OutputKey === outputLogicalId);

        if (output === undefined) {
          const errorMessage = [
            'Trying to request a non exported variable from CloudFormation.',
            ` Stack name: "${stackName}"`,
            ` Requested variable: "${outputLogicalId}".`,
          ].join('');
          return BbPromise.reject(new this.serverless.classes.Error(errorMessage));
        }
        return BbPromise.resolve(output.OutputValue);
      });
  }