Web App Deployment Slot doesn't turn on System Assigned Identity

[Deploy a Web App with custom deployment slots](https://github.com/Azure/azure-quickstart-templates/tree/master/101-webapp-custom-deployment-slots)
### Issue Details
When "clonningInfo" is specified in the properties, it won't create a system assigned identity even it is specified in the template. This only happens if clonningInfo is specified. Below is the template snippet:
```json
{
"type": "Microsoft.Web/sites/slots",
"apiVersion": "2018-11-01",
"name": "[concat(parameters('name'), '/', parameters('deploymentSlots')[copyIndex()])]",
"kind": "app",
"location": "[parameters('location')]",
"identity": {
"type": "SystemAssigned"
},
"comments": "This specifies the web app slots.",
"tags": "[parameters('commonResourceTags')]",
"properties": {
"serverFarmId": "[resourceId('Microsoft.Web/serverfarms', parameters('hostingPlanName'))]",
"cloningInfo": {
"sourceWebAppId": "[resourceId('Microsoft.Web/Sites/', parameters('name'))]"
}
},
"dependsOn": [
"[resourceId('Microsoft.Web/Sites', parameters('name'))]"
],
"copy": {
"name": "copySlots",
"count": "[length(parameters('deploymentSlots'))]"
}
}
```

### Repro steps (*if necessary, delete otherwise*)
1. Create an app service azure resource with deployment slot that clones the information to the deployment slot and at the same time assign an SystemAssigned identity

Start with the 101-webapp-custom-deployment-slots template linked in the issue and compare the deployment-slot resource with the provided JSON. Reproduce a slot deployment using cloningInfo and SystemAssigned identity, then verify that the slot receives the identity; the issue is done when this behavior works as described.

Written by the indexing model from the issue text.