fix: opt out of 'Anitimalware Signatures SDP' extension for Windows VHDs (#4888)

Signed-off-by: Mark Rossetti <marosset@microsoft.com>

vhd/packer/windows-vhd-builder.json

@@ -44,7 +44,8 @@
             "azure_tags": {
                 "os": "Windows",
                 "now": "{{user `create_time`}}",
-                "createdBy": "aks-engine-vhd-pipeline"
+                "createdBy": "aks-engine-vhd-pipeline",
+                "SkipASMAV": "true"
             }
         }
     ],