Add primary field to ip_configuration required by Azure

* Required by terraform-provider-azurerm v1.17+
* https://github.com/terraform-providers/terraform-provider-azurerm/pull/2035

CHANGES.md

@@ -16,6 +16,10 @@ Notable changes between versions.
 
 * Add `disk_iops` variable for EBS volume IOPS ([#314](https://github.com/poseidon/typhoon/pull/314))
 
+#### Azure
+
+* Add `primary` field to `ip_configuration`. Required by `terraform-provider-azurerm` v1.17+ ([#331](https://github.com/poseidon/typhoon/pull/331))
+
 #### Addons
 
 * Configure Heapster to scrape Kubelets with bearer token auth ([#323](https://github.com/poseidon/typhoon/pull/323))

azure/container-linux/kubernetes/workers/workers.tf

@@ -58,6 +58,7 @@ resource "azurerm_virtual_machine_scale_set" "workers" {
 
     ip_configuration {
       name      = "ip0"
+      primary   = true
       subnet_id = "${var.subnet_id}"
 
       # backend address pool to which the NIC should be added