Task: Use TaskEvent messages

Task registry messages make more sense to use
for task events then standard registry entries
when applicable. Use them.

Tested:

    "Messages": [
        {
            "@odata.type": "#Message.v1_0_0.Message",
            "Message": "The task with id 0 has started.",
            "MessageArgs": [
                "0"
            ],
            "MessageId": "TaskEvent.1.0.1.TaskStarted",
            "Resolution": "None.",
            "Severity": "OK"
        }
    ],

Validator passed

Change-Id: I707492544e18def2833e8a2e2216ce803c42c775
Signed-off-by: James Feist <james.feist@linux.intel.com>
diff --git a/redfish-core/lib/log_services.hpp b/redfish-core/lib/log_services.hpp
index 667fb40..79433b9 100644
--- a/redfish-core/lib/log_services.hpp
+++ b/redfish-core/lib/log_services.hpp
@@ -2274,7 +2274,9 @@
                    const std::shared_ptr<task::TaskData> &taskData) {
                     if (!err)
                     {
-                        taskData->messages.emplace_back(messages::success());
+                        taskData->messages.emplace_back(
+                            messages::taskCompletedOK(
+                                std::to_string(taskData->index)));
                         taskData->state = "Completed";
                     }
                     return task::completed;