Add node_labels variable in workers modules to set initial node labels (#550)

* Also add `worker_node_labels` variable in `kubernetes` modules to set
initial node labels for the default workers

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

@@ -105,3 +105,8 @@ variable "cluster_domain_suffix" {
   default = "cluster.local"
 }
 
+variable "node_labels" {
+  description = "List of additional labels to add to worker nodes"
+  type = list
+  default = []
+}