add x-ms-enum for resourcehealth (Azure#17425)

Author: ArcturusZhang

specification/resourcehealth/resource-manager/Microsoft.ResourceHealth/stable/2017-07-01/resourcehealth.json

@@ -966,6 +966,10 @@
       "enum": [
         "default"
       ],
+      "x-ms-enum": {
+        "name": "IssueName",
+        "modelAsString": false
+      },
       "x-ms-parameter-location": "method"
     },
     "ResourceGroupNameParameter": {