Change discrete threshold value to string

For compliance with redfish schema, trigger with discrete
thresholds should use string as a value - instead of double.
This will impact dbus method "AddTrigger" for Trigger interface.
As there are currently no known sensors with discrete values,
telemetry service will still treat the value as a double internally.

Tested:
 - UT passed.
 - Trigger with discrete threshold value was successfully created
   using dbus call.
 - local redfish-tests for trigger persistency are passing after
   type modification.

Signed-off-by: Szymon Dompke <szymon.dompke@intel.com>
Change-Id: I8201ce5e2a45647206c5ccd85fc45ea8f32961af
diff --git a/src/interfaces/trigger_types.hpp b/src/interfaces/trigger_types.hpp
index 568756f..6721e68 100644
--- a/src/interfaces/trigger_types.hpp
+++ b/src/interfaces/trigger_types.hpp
@@ -39,13 +39,13 @@
     return std::string(utils::enumToString(details::convDataSeverity, v));
 }
 
-using ThresholdParam = std::tuple<std::string, std::string, uint64_t, double>;
+using ThresholdParam =
+    std::tuple<std::string, std::string, uint64_t, std::string>;
 
-using LabeledThresholdParam =
-    utils::LabeledTuple<std::tuple<std::string, Severity, uint64_t, double>,
-                        utils::tstring::UserId, utils::tstring::Severity,
-                        utils::tstring::DwellTime,
-                        utils::tstring::ThresholdValue>;
+using LabeledThresholdParam = utils::LabeledTuple<
+    std::tuple<std::string, Severity, uint64_t, std::string>,
+    utils::tstring::UserId, utils::tstring::Severity, utils::tstring::DwellTime,
+    utils::tstring::ThresholdValue>;
 } // namespace discrete
 
 namespace numeric