Revert "data/aws: Switch to m4.large"

This reverts commit a230376
(data/aws: Switch to m4.large, 2018-11-29, openshift#765), now that we've
bumped our t3.medium limits in the CI and dev accounts to cover our
expected loads.  Moving from m4.large to t3.medium also reduces memory
from 8 GiB [1] to 4 GiB [2], but after a recent run of end-to-end
tests, the master consumption was:

  $ free -m
                total        used        free      shared  buff/cache   available
  Mem:           7980        2263         794           8        4922        5259
  Swap:             0           0           0

so 4 GiB should be sufficient.  And it also matches our libvirt setup
since e59513f (libvirt: Add Terraform variables for memory/CPU, bump
master to 4GiB, 2018-12-04, openshift#785).

[1]: https://aws.amazon.com/ec2/instance-types/#General_Purpose
[2]: https://aws.amazon.com/ec2/instance-types/t3/#Product_Details

data/data/aws/bootstrap/variables.tf

@@ -26,7 +26,7 @@ variable "ignition" {
 
 variable "instance_type" {
   type        = "string"
-  default     = "m4.large"
+  default     = "t3.medium"
   description = "The EC2 instance type for the bootstrap node."
 }
 

data/data/aws/variables-aws.tf

@@ -9,10 +9,10 @@ EOF
 
 variable "aws_master_ec2_type" {
   type        = "string"
-  description = "Instance size for the master node(s). Example: `m4.large`."
+  description = "Instance size for the master node(s). Example: `t3.medium`."
 
   # FIXME: get this wired up to the machine default
-  default = "m4.large"
+  default = "t3.medium"
 }
 
 variable "aws_ec2_ami_override" {

pkg/asset/machines/worker.go

@@ -27,7 +27,7 @@ import (
 
 func defaultAWSMachinePoolPlatform() awstypes.MachinePool {
 	return awstypes.MachinePool{
-		InstanceType: "m4.large",
+		InstanceType: "t3.medium",
 	}
 }