Allow custom initial node taints on worker pool nodes

* Add `node_taints` variable to worker modules to set custom
initial node taints on cloud platforms that support auto-scaling
worker pools of heterogeneous nodes (i.e. AWS, Azure, GCP)
* Worker pools could use custom `node_labels` to allowed workloads
to select among differentiated nodes, while custom `node_taints`
allows a worker pool's nodes to be tainted as special to prevent
scheduling, except by workloads that explicitly tolerate the
taint
* Expose `daemonset_tolerations` in AWS, Azure, and GCP kubernetes
cluster modules, to determine whether `kube-system` components
should tolerate the custom taint (advanced use covered in docs)

Rel: #550, #663
Closes #429

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

@@ -86,6 +86,7 @@ data "template_file" "worker-config" {
     cluster_dns_service_ip = cidrhost(var.service_cidr, 10)
     cluster_domain_suffix  = var.cluster_domain_suffix
     node_labels            = join(",", var.node_labels)
+    node_taints            = join(",", var.node_taints)
   }
 }