Investigate and document how to stop an Azure AKS K8S cluster

[hjhsalo]

Since AKS K8S cluster's system node pool cannot be brought down without destroying the whole cluster we should investigate and document how AKS cluster can be stopped in order to reduce costs (actual €s)
(#3 (comment))
(#11 )

https://docs.microsoft.com/en-us/azure/aks/start-stop-cluster
https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs/resources/kubernetes_cluster_node_pool

This should be thought trough carefully so that Terraform state handling still works even if AKS cluster has been turned off.