add x-ms-enum to trafficmanager (Azure#17511)

Author: ArcturusZhang

specification/trafficmanager/resource-manager/Microsoft.Network/stable/2018-08-01/trafficmanager.json

@@ -781,6 +781,10 @@
             "enum": [
               "default"
             ],
+            "x-ms-enum": {
+              "name": "HeatMapType",
+              "modelAsString": false
+            },
             "description": "The type of HeatMap for the Traffic Manager profile."
           },
           {