async function getValue(req, res) {
    const value = await gladys.variable.getValue(req.params.variable_key);
    if (!value) {
      throw new NotFoundError('VARIABLE_NOT_FOUND');
    }
    res.json({ value });
  }
@api {get} /api/variable/:variable_key Get variable
@apiName getVariable
@apiGroup Variable
@apiParam {string} value value to save