Add Deassert Event to PSU

Tested:
Plug out AC cable and plug in it again, below redfish log is created
{
    "@odata.context": "/redfish/v1/$metadata#LogEntry.LogEntry",
    "@odata.id": "/redfish/v1/Systems/system/LogServices/EventLog/Entries/
1566427243",
    "@odata.type": "#LogEntry.v1_4_0.LogEntry",
    "Created": "2019-08-21T22:40:43+00:00",
    "EntryType": "Event",
    "Id": "1566427243",
    "Message": "Power supply PSU2 AC power regained.",
    "MessageArgs": [
        "PSU2"
    ],
    "MessageId": "OpenBMC.0.1.PowerSupplyACInserted",
    "Name": "System Event Log Entry",
    "Severity": "OK"
},

Signed-off-by: Cheng C Yang <cheng.c.yang@linux.intel.com>
Change-Id: Ic2626c99f06962dafabb580fe498fb558480f61e
diff --git a/include/PSUEvent.hpp b/include/PSUEvent.hpp
index 104b910..165ac92 100644
--- a/include/PSUEvent.hpp
+++ b/include/PSUEvent.hpp
@@ -49,7 +49,8 @@
     static constexpr size_t warnAfterErrorCount = 10;
     std::string psuName;
     std::string fanName;
-    std::string messageID = "";
+    std::string assertMessage;
+    std::string deassertMessage;
 };
 
 class PSUCombineEvent