Add disk_type variable for EBS volume type on AWS

* Change EBS volume type from `standard` ("prior generation)
 to `gp2`. Prometheus alerts are tuned for SSDs
* Other platforms have fast enough disks by default

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

@@ -42,7 +42,7 @@ resource "aws_launch_configuration" "worker" {
 
   # storage
   root_block_device {
-    volume_type = "standard"
+    volume_type = "${var.disk_type}"
     volume_size = "${var.disk_size}"
   }