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/cl/worker.yaml

@@ -73,6 +73,9 @@ systemd:
           %{~ for label in split(",", node_labels) ~}
           --node-labels=${label} \
           %{~ endfor ~}
+          %{~ for taint in split(",", node_taints) ~}
+          --register-with-taints=${taint} \
+          %{~ endfor ~}
           --pod-manifest-path=/etc/kubernetes/manifests \
           --provider-id=aws:///$${COREOS_EC2_AVAILABILITY_ZONE}/$${COREOS_EC2_INSTANCE_ID} \
           --read-only-port=0 \