Address issues in https://github.com/clong/DetectionLab/issues/216 by utilizing Terraform-native options

Terraform/variables.tf

@@ -1,6 +1,10 @@
 variable "region" {
   default = "us-west-1"
 }
+variable "availability_zone" {
+  description = "https://www.terraform.io/docs/providers/aws/d/availability_zone.html"
+  default = ""
+}
 variable "shared_credentials_file" {
   description = "Path to your AWS credentials file"
   type = "string"
@@ -25,6 +29,11 @@ variable "ip_whitelist" {
   type = "list"
   default = [""]
 }
+variable "external_dns_servers" {
+  description = "Configure lab to allow external DNS resolution"
+  type = "list"
+  default = ["8.8.8.8"]
+}
 
 # The logger host will provision itself and does not use a pre-built AMI