The SystemAssigned managed identity requires a preview feature to be
enabled.
```
rgil@rem5:~/atst/terraform/providers/dev$ az feature list|grep MSIPreview
"id": "/subscriptions/95934d54-980d-47cc-9bce-3a96bf9a2d1b/providers/Microsoft.Features/providers/Microsoft.ContainerService/features/MSIPreview",
"name": "Microsoft.ContainerService/MSIPreview",
rgil@rem5:~/atst/terraform/providers/dev$ az feature register --namespace Microsoft.ContainerService --name MSIPreview
Once the feature 'MSIPreview' is registered, invoking 'az provider register -n Microsoft.ContainerService' is required to get the change propagated
{
"id": "/subscriptions/95934d54-980d-47cc-9bce-3a96bf9a2d1b/providers/Microsoft.Features/providers/Microsoft.ContainerService/features/MSIPreview",
"name": "Microsoft.ContainerService/MSIPreview",
"properties": {
"state": "Registering"
},
"type": "Microsoft.Features/providers/features"
}
rgil@rem5:~/atst/terraform/providers/dev$ az provider register -n Microsoft.ContainerService
rgil@rem5:~/atst/terraform/providers/dev$
```
This also now integrates the policy for keyvault with the k8s managed
identity (system assigned).
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