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/variables.tf

@@ -46,6 +46,12 @@ variable "disk_size" {
   description = "Size of the EBS volume in GB"
 }
 
+variable "disk_type" {
+  type        = "string"
+  default     = "gp2"
+  description = "Type of the EBS volume (e.g. standard, gp2, io1)"
+}
+
 variable "clc_snippets" {
   type        = "list"
   description = "Container Linux Config snippets"