lib/classes/Variables.js/Variables/getValueStrToBool
class Variables:  getValueStrToBool(variableString) {
    return BbPromise.try(() => {
      if (_.isString(variableString)) {
        const groups = variableString.match(this.strToBoolRefSyntax);
        const param = groups[1].trim();
        if (/^(true|false|0|1)$/.test(param)) {
          if (param === 'false' || param === '0') {
            return false;
          }
          // truthy or non-empty strings
          return true;
        }
        throw new this.serverless.classes.Error(
          'Unexpected strToBool input; expected either "true", "false", "0", or "1".'
        );
      }
      // Cast non-string inputs
      return Boolean(variableString);
    });
  }