feat: configure adding default node pool upgrade settings

Using a current azurerm without explicit upgrade settings will remove
default upgrade settings:

Terraform will perform the following actions:
  ~ resource "azurerm_kubernetes_cluster" "k8s" {
      ~ default_node_pool {
          - upgrade_settings {
              - drain_timeout_in_minutes      = 0 -> null
              - max_surge                     = "10%" -> null
              - node_soak_duration_in_minutes = 0 -> null
            }
        }
    }
Plan: 0 to add, 1 to change, 0 to destroy.

This feature makes adding upgrade settings configurable

Author: kpshjk

README.md

@@ -190,6 +190,22 @@ Type: `string`
 
 Default: `"default"`
 
+### default\_node\_pool\_upgrade\_settings\_enabled
+
+Description: default upgrade settings is added to default node pool
+
+Type: `boolean`
+
+Default: `false`
+
+### default\_node\_pool\_upgrade\_settings\_max\_surge
+
+Description: max surge of upgrade settings for default node pool
+
+Type: `string`
+
+Default: `"10%"`
+
 ### dns\_prefix
 
 Description: DNS-Prefix to use. Defaults to cluster name

main.tf

@@ -12,6 +12,9 @@ locals {
   has_automatic_channel_upgrade_maintenance_window = var.automatic_upgrade_channel != "none" ? [
     var.automatic_upgrade_channel
   ] : []
+  has_default_node_pool_upgrade_settings = var.default_node_pool_upgrade_settings_enabled == true ? [
+    var.default_node_pool_upgrade_settings_enabled
+  ] : []
 }
 
 # Log analytics required for OMS Agent result processing - usually other logging solutions are used. Hence the affected tfsec rule is
@@ -61,6 +64,12 @@ resource "azurerm_kubernetes_cluster" "k8s" {
     auto_scaling_enabled        = var.auto_scaling_enabled
     min_count                   = var.auto_scaling_min_node_count
     max_count                   = var.auto_scaling_max_node_count
+    dynamic "upgrade_settings" {
+      for_each = local.has_default_node_pool_upgrade_settings
+      content {
+        max_surge               = var.default_node_pool_upgrade_settings_max_surge
+      }
+    }
   }
 
   dynamic "api_server_access_profile" {

vars.tf

@@ -270,3 +270,21 @@ variable "maintenance_window_auto_upgrade_utc_offset" {
     see https://learn.microsoft.com/en-us/azure/aks/planned-maintenance#creating-a-maintenance-window
   EOF
 }
+
+variable "default_node_pool_upgrade_settings_enabled" {
+  type        = bool
+  default     = false
+  description = <<-EOF
+    Values:
+    false, true
+  EOF
+}
+
+variable "default_node_pool_upgrade_settings_max_surge" {
+  type        = string
+  default     = "10%"
+  description = <<-EOF
+    Example: "10%"
+    see https://learn.microsoft.com/en-us/azure/aks/upgrade-aks-cluster?tabs=azure-cli#customize-node-surge-upgrade
+  EOF
+}