Missing field for Domain Join

[azuredeploy.json](https://github.com/Azure/azure-quickstart-templates/blob/master/quickstarts/microsoft.compute/vm-domain-join-existing/azuredeploy.json)

### There is no data field within the template to enter in the description for the computer account/object within AD, just its name, dns name, and OU:

{
"apiVersion": "2015-06-15",
"type": "Microsoft.Compute/virtualMachines/extensions",
"name": "[concat(parameters('virtualMachineComputerName'),'/joindomain')]",
"location": "[parameters('location')]",
"dependsOn": [
"[concat('Microsoft.Compute/virtualMachines/', parameters('virtualMachineComputerName'))]"
],
"properties": {
"publisher": "Microsoft.Compute",
"type": "JsonADDomainExtension",
"typeHandlerVersion": "1.3",
"autoUpgradeMinorVersion": true,
"settings": {
"Name": "[parameters('domainToJoin')]",
"OUPath": "[parameters('ouPath')]",
"User": "[concat(parameters('domainToJoin'), '\\', parameters('domainUsername'))]",
"Restart": "true",
"Options": "[parameters('domainJoinOptions')]"
},
"protectedSettings": {
"Password": "[parameters('domainPassword')]"
}
}
},

Start with quickstarts/microsoft.compute/vm-domain-join-existing/azuredeploy.json and review the existing domain-join parameters and JsonADDomainExtension settings. Add a template input for the computer account description and ensure the value is passed to the domain-join configuration; done means the template exposes and uses that field alongside the existing name, DNS name, and OU inputs.

Written by the indexing model from the issue text.