Lower the number of availability zones used.

I don't care all that much about HA in this setup and I do wish to save
a few bucks when I can, but I do need to AZs in some cases so 2 AZs is
to the way to go.

Terraform/AWS/vpc.tf

@@ -66,7 +66,7 @@ output "internet_gateway_name" {
 }
 
 variable "subnet_count" {
-  default     = 3
+  default     = 2
   description = "Number of each private and public subnets."
   type        = number
 }