Rename worker pool modules' count variable to worker_count

* This change affects users who use worker pools on AWS, GCP, or
Azure with a Container Linux derivative
* Rename worker pool modules' `count` variable to `worker_count`,
because `count` will be a reserved variable name in Terraform v0.12

aws/container-linux/kubernetes/workers.tf

@@ -6,7 +6,7 @@ module "workers" {
   vpc_id          = "${aws_vpc.network.id}"
   subnet_ids      = ["${aws_subnet.public.*.id}"]
   security_groups = ["${aws_security_group.worker.id}"]
-  count           = "${var.worker_count}"
+  worker_count    = "${var.worker_count}"
   instance_type   = "${var.worker_type}"
   os_image        = "${var.os_image}"
   disk_size       = "${var.disk_size}"

aws/container-linux/kubernetes/workers/variables.tf

@@ -22,7 +22,7 @@ variable "security_groups" {
 
 # instances
 
-variable "count" {
+variable "worker_count" {
   type        = "string"
   default     = "1"
   description = "Number of instances"

aws/container-linux/kubernetes/workers/workers.tf

@@ -3,9 +3,9 @@ resource "aws_autoscaling_group" "workers" {
   name = "${var.name}-worker ${aws_launch_configuration.worker.name}"
 
   # count
-  desired_capacity          = "${var.count}"
-  min_size                  = "${var.count}"
-  max_size                  = "${var.count + 2}"
+  desired_capacity          = "${var.worker_count}"
+  min_size                  = "${var.worker_count}"
+  max_size                  = "${var.worker_count + 2}"
   default_cooldown          = 30
   health_check_grace_period = 30