Add quickstart sample for private AKS cluster with custom DNS

quickstart/301-aks-private-cluster/README.md

@@ -0,0 +1,55 @@
+# Private Azure Kubernetes Service with Custom DNS Server
+
+This template deploys a private Azure Kubernetes Service cluster configured with custom DNS server. A new VNet with provided
+custom DNS server would be provisioned and the AKS cluster is deployed into this new VNet. The DNS server's VNet would
+also be linked to AKS provisioned private DNS zone, so that AKS cluster's private FQDN could be resolved successfully on
+custom DNS server.
+
+To use this template, ensure the following pre-requirements have been set:
+
+* Azure CLI and terraform installed locally
+* Pre-configure DNS servers outside of AKS VNet
+* Forward AKS cluster FQDN `azmk8s.io` (or only private cluster FQDN `privatelink.<region>.azmk8s.io`) to Azure DNS `168.63.129.16`
+* Get the DNS servers IP address, which would be set in `custom_dns`
+* Get the DNS server's VNet resource ID, which would be set in `custom_dns_vnet_id`
+
+## Resources
+
+| Terraform Resource Type | Description |
+|-------------------------|-------------|
+| `azurerm_resource_group` | The resource group all resources are deployed into |
+| `azurerm_virtual_network` | The VNet that AKS cluster would be deployed on |
+| `azurerm_subnet` | The subnet that AKS cluster would be deployed on |
+| `azurerm_kubernetes_cluster` | The AKS cluster |
+| `null_resource.dns_zone_link` | Link custom DNS server's VNet to AKS private DNS zone|
+
+## Variables
+
+| Name | Description |
+|------|-------------|
+| `resource_group_name` | Name of the Azure resource group|
+| `cluster_name` | Name of the AKS cluster|
+| `custom_dns` | IP of custom DNS server|
+| `custom_dns_vnet_id` |Resource ID of the Azure VNet that holds custom DNS server|
+| `client_id` | The service principal ID|
+| `client_secret` | The service principal password|
+| `agent_count` | The number of K8S nodes to provision|
+| `kubernetes_version` | The version of K8S to provision|
+| `ssh_public_key` | The SSH public key of K8S nodes |
+| `dns_prefix` | The DNS prefix of AKS cluster |
+| `location`  | The location of Azure resources |
+
+## Usage
+
+```sh
+terraform plan \
+    -var 'resource_group_name=aks-quickstart' \
+    -var 'cluster_name=aks' \
+    -var 'custom_dns=<your-custom-dns-IP>' \
+    -var 'custom_dns_vnet_id=<your-custom-dns-vnet-id>' \
+    -var 'client_id=<your-client-id>' \
+    -var 'client_secret=<your-client-secret>' \
+    -out demo.tfplan
+
+terraform apply demo.tfplan
+```