virtualNetworkName is needed for vmss masters (#4159)

parts/k8s/kubernetesmastervars.t

@@ -193,8 +193,8 @@
     {{else}}
     "subnetName": "[concat(parameters('orchestratorName'), '-subnet')]",
     "vnetSubnetID": "[concat(variables('vnetID'),'/subnets/',variables('subnetName'))]",
-    "virtualNetworkName": "[concat(parameters('orchestratorName'), '-vnet-', parameters('nameSuffix'))]",
     {{end}}
+    "virtualNetworkName": "[concat(parameters('orchestratorName'), '-vnet-', parameters('nameSuffix'))]",
     "vnetID": "[resourceId('Microsoft.Network/virtualNetworks',variables('virtualNetworkName'))]",
     "virtualNetworkResourceGroupName": "''",
   {{end}}