udpated test instance from t2 to t3

[colinedwardwood]

No description provided.

[ericcalabretta]

@colinedwardwood @kenlangdon

I think this is an unused variable and we can just remove it. I don't see a reference to it anywhere, also the only t2.micro I see is in a demo_instances.tf.example file which we can also remove.

variable "test_instance_type" {
  default = "t2.micro"
}```


This example 
https://github.com/chef-cft/national-parks-demo/blob/master/terraform/chef-automate/aws/demo_instances.tf.example