Change AWS default type from t2.small to t3.small

* T3 is the next generation general purpose burstable
instance type. Compared with t2.small, the t3.small is
cheaper, has 2 vCPU (instead of 1) and provides 5 Gbps
of pod-to-pod bandwidth (instead of 1 Gbps)

aws/container-linux/kubernetes/variables.tf

@@ -31,13 +31,13 @@ variable "worker_count" {
 
 variable "controller_type" {
   type        = "string"
-  default     = "t2.small"
+  default     = "t3.small"
   description = "EC2 instance type for controllers"
 }
 
 variable "worker_type" {
   type        = "string"
-  default     = "t2.small"
+  default     = "t3.small"
   description = "EC2 instance type for workers"
 }