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

azure/flatcar-linux/kubernetes/workers/variables.tf

@@ -94,6 +94,12 @@ variable "node_labels" {
   default     = []
 }
 
+variable "node_taints" {
+  type        = list(string)
+  description = "List of initial node taints"
+  default     = []
+}
+
 # unofficial, undocumented, unsupported
 
 variable "cluster_domain_suffix" {