fix(aks): Fix the way that subnets are handled by the node pools

[apetre070]

Fixes the following error on nightly integration tests:

│ Error: Subnet (Subscription: "***"
│ Resource Group Name: "network-ws64b8cd"
│ Virtual Network Name: "network-ws64b8cd"
│ Subnet Name: "private-3-ws64b8cd") was not found
│ 
│   with module.main.data.azurerm_subnet.private["3"],
│   on ../../../../../.oci_extracted/ghcr.io-windsorcli/core-latest/terraform/cluster/azure-aks/main.tf line 37, in data "azurerm_subnet" "private":
│   37: data "azurerm_subnet" "private" {
│ 
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│ Error: Subnet (Subscription: "***"
│ Resource Group Name: "network-ws64b8cd"
│ Virtual Network Name: "network-ws64b8cd"
│ Subnet Name: "private-2-ws64b8cd") was not found
│ 
│   with module.main.data.azurerm_subnet.private["2"],
│   on ../../../../../.oci_extracted/ghcr.io-windsorcli/core-latest/terraform/cluster/azure-aks/main.tf line 37, in data "azurerm_subnet" "private":
│   37: data "azurerm_subnet" "private" {
│ 
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