modules/azure: Enable the use of external master & worker subnets

- Only create master & worker subnets if no external vnets exist
- The `join()` interpolation function is used to work around
hashicorp/hil#50 when the subnets are conditionally
created. For more detail, see:
coreos@7ab31b0)

Documentation/variables/azure.md

@@ -10,9 +10,11 @@ This document gives an overview of the variables used in the different platforms
 | tectonic_azure_create_dns_zone | If set to true, create an Azure DNS zone | string | `true` |
 | tectonic_azure_dns_resource_group |  | string | `tectonic-dns-group` |
 | tectonic_azure_etcd_vm_size | Instance size for the etcd node(s). Example: Standard_DS2_v2. | string | `Standard_DS2_v2` |
+| tectonic_azure_external_master_subnet_id | Subnet id within an existing VNet to deploy master nodes into. Required to use an existing VNet. Example: the subnet ID starts with `"/subscriptions/{subscriptionId}"` or `"/providers/{resourceProviderNamespace}"'`. | string | `` |
 | tectonic_azure_external_rsg_name | Pre-existing resource group to use as parent for cluster resources. | string | `` |
 | tectonic_azure_external_vnet_id | ID of an existing Virtual Network to launch nodes into. Example: VNet1. Leave blank to create a new Virtual Network. | string | `` |
 | tectonic_azure_external_vnet_name | Pre-existing virtual network to create cluster into. | string | `` |
+| tectonic_azure_external_worker_subnet_id | Subnet id within an existing VNet to deploy worker nodes into. Required to use an existing VNet. Example: the subnet ID starts with `"/subscriptions/{subscriptionId}"` or `"/providers/{resourceProviderNamespace}"'`. | string | `` |
 | tectonic_azure_image_reference | The image ID as given in `azure image list`. Specifies the OS image of the VM. | map | `<map>` |
 | tectonic_azure_location |  | string | - |
 | tectonic_azure_master_vm_size | Instance size for the master node(s). Example: Standard_DS2_v2. | string | `Standard_DS2_v2` |

examples/terraform.tfvars.azure

@@ -14,6 +14,9 @@ tectonic_azure_dns_resource_group = "tectonic-dns-group"
 // Instance size for the etcd node(s). Example: Standard_DS2_v2.
 tectonic_azure_etcd_vm_size = "Standard_DS2_v2"
 
+// Subnet id within an existing VNet to deploy master nodes into. Required to use an existing VNet. Example: the subnet ID starts with `"/subscriptions/{subscriptionId}"` or `"/providers/{resourceProviderNamespace}"'`.
+tectonic_azure_external_master_subnet_id = ""
+
 // Pre-existing resource group to use as parent for cluster resources.
 tectonic_azure_external_rsg_name = ""
 
@@ -23,6 +26,9 @@ tectonic_azure_external_vnet_id = ""
 // Pre-existing virtual network to create cluster into.
 tectonic_azure_external_vnet_name = ""
 
+// Subnet id within an existing VNet to deploy worker nodes into. Required to use an existing VNet. Example: the subnet ID starts with `"/subscriptions/{subscriptionId}"` or `"/providers/{resourceProviderNamespace}"'`.
+tectonic_azure_external_worker_subnet_id = ""
+
 // 
 tectonic_azure_image_reference = ""
 

modules/azure/vnet/outputs.tf

@@ -2,13 +2,10 @@ output "vnet_id" {
   value = "${var.external_vnet_name == "" ? join("",azurerm_virtual_network.tectonic_vnet.*.name) : var.external_vnet_name }"
 }
 
-# We have to do this join() & split() 'trick' because null_data_source and 
-# the ternary operator can't output lists or maps
-#
 output "master_subnet" {
-  value = "${azurerm_subnet.master_subnet.id}"
+  value = "${var.external_vnet_name == "" ?  join(" ", azurerm_subnet.master_subnet.*.id) : var.external_master_subnet_id }"
 }
 
 output "worker_subnet" {
-  value = "${azurerm_subnet.worker_subnet.id}"
+  value = "${var.external_vnet_name == "" ?  join(" ", azurerm_subnet.worker_subnet.*.id) : var.external_worker_subnet_id }"
 }

modules/azure/vnet/variables.tf

@@ -12,16 +12,6 @@ variable "tectonic_cluster_name" {
   type = "string"
 }
 
-variable "tectonic_azure_external_vnet_master_subnets" {
-  type    = "list"
-  default = []
-}
-
-variable "tectonic_azure_external_vnet_worker_subnets" {
-  type    = "list"
-  default = []
-}
-
 variable "resource_group_name" {
   type = "string"
 }
@@ -38,3 +28,13 @@ variable "external_vnet_name" {
   type    = "string"
   default = ""
 }
+
+variable "external_master_subnet_id" {
+  type    = "string"
+  default = ""
+}
+
+variable "external_worker_subnet_id" {
+  type    = "string"
+  default = ""
+}

modules/azure/vnet/virtualnet.tf

@@ -11,13 +11,15 @@ resource "azurerm_virtual_network" "tectonic_vnet" {
 }
 
 resource "azurerm_subnet" "master_subnet" {
+  count                = "${var.external_vnet_name == "" ? 1 : 0}"
   name                 = "${var.tectonic_cluster_name}_master_subnet"
   resource_group_name  = "${var.resource_group_name}"
   virtual_network_name = "${var.external_vnet_name == "" ? join("",azurerm_virtual_network.tectonic_vnet.*.name) : var.external_vnet_name }"
   address_prefix       = "${cidrsubnet(var.vnet_cidr_block, 4, 0)}"
 }
 
 resource "azurerm_subnet" "worker_subnet" {
+  count                = "${var.external_vnet_name == "" ? 1 : 0}"
   name                 = "${var.tectonic_cluster_name}_worker_subnet"
   resource_group_name  = "${var.resource_group_name}"
   virtual_network_name = "${var.external_vnet_name == "" ? join("",azurerm_virtual_network.tectonic_vnet.*.name) : var.external_vnet_name }"

platforms/azure/main.tf

@@ -14,6 +14,8 @@ module "vnet" {
   tectonic_cluster_name = "${var.tectonic_cluster_name}"
   vnet_cidr_block       = "${var.tectonic_azure_vnet_cidr_block}"
   external_vnet_name    = "${var.tectonic_azure_external_vnet_name}"
+  external_master_subnet_id = "${var.tectonic_azure_external_master_subnet_id}"
+  external_worker_subnet_id = "${var.tectonic_azure_external_worker_subnet_id}"
 }
 
 module "etcd" {

platforms/azure/variables.tf

@@ -86,3 +86,15 @@ variable "tectonic_azure_create_dns_zone" {
   description = "If set to true, create an Azure DNS zone"
   default     = "true"
 }
+
+variable "tectonic_azure_external_master_subnet_id" {
+  type        = "string"
+  description = "Subnet id within an existing VNet to deploy master nodes into. Required to use an existing VNet. Example: the subnet ID starts with `\"/subscriptions/{subscriptionId}\"` or `\"/providers/{resourceProviderNamespace}\"'`. "
+  default     = ""
+}
+
+variable "tectonic_azure_external_worker_subnet_id" {
+  type        = "string"
+  description = "Subnet id within an existing VNet to deploy worker nodes into. Required to use an existing VNet. Example: the subnet ID starts with `\"/subscriptions/{subscriptionId}\"` or `\"/providers/{resourceProviderNamespace}\"'`. "
+  default     = ""
+}