Add worker group option to protect from scale in. (#135)

See [#134](https://github.com/terraform-aws-modules/terraform-aws-eks/issues/134)

local.tf

@@ -29,6 +29,7 @@ locals {
     subnets                       = "${join(",", var.subnets)}"     # A comma delimited string of subnets to place the worker nodes in. i.e. subnet-123,subnet-456,subnet-789
     autoscaling_enabled           = false                           # Sets whether policy and matching tags will be added to allow autoscaling.
     additional_security_group_ids = ""                              # A comman delimited list of additional security group ids to include in worker launch config
+    protect_from_scale_in         = false                           # Prevent AWS from scaling in, so that cluster-autoscaler is solely responsible.
   }
 
   workers_group_defaults = "${merge(local.workers_group_defaults_defaults, var.workers_group_defaults)}"