Create a separate user node pool for AKS K8S cluster

[hjhsalo]

DON'T START WORKING ON THIS YET
This needs to be thought trough in detail; i.e. we need to do development cost / benefit analysis.

terraform apply -var="k8s_agent_count=0" (see #3 (comment)) doesn't work since AKS separates node pools to a system pool and user pool(s).

See these for more info
https://docs.microsoft.com/en-us/azure/aks/use-system-pools#system-and-user-node-pools
https://docs.microsoft.com/en-us/azure/aks/use-multiple-node-pools

Currently k8s_agent_count (https://github.com/hjhsalo/smad-terraform-azure/blob/97363bd33c6a052b73f2c85e05e165d32fd6b757/modules/k8s/main.tf#L71-L72) affects system pool and it must be at least 1.

We could define a separate user pool which can be shutdown by supplying node_count=0.

[msrn]

This can be done with azurerm_kubernetes_cluster_node_pool