Add worker bootstrap-node-taints setting

The control-plane charm already allows defining node taints. We'll
add the same for worker nodes.

charms/worker/charmcraft.yaml

@@ -60,6 +60,19 @@ bases:
 
 config:
   options:
+    bootstrap-node-taints:
+      type: string
+      default: ""
+      description: |
+        Space-separated list of taints to apply to this node at registration time.
+
+        This config is only used at bootstrap time when Kubelet first registers the
+        node with Kubernetes. To change node taints after deploy time, use kubectl
+        instead.
+
+        For more information, see the upstream Kubernetes documentation about
+        taints:
+        https://kubernetes.io/docs/concepts/scheduling-eviction/taint-and-toleration/
     node-labels:
       default: ""
       type: string

charms/worker/k8s/src/config/extra_args.py

@@ -70,3 +70,13 @@ def craft(
 
     cmd = _parse(src["kubelet-extra-args"])
     dest.extra_node_kubelet_args = cmd
+
+    is_worker_node = (
+        isinstance(dest, NodeJoinConfig) and not
+        isinstance(dest, ControlPlaneNodeJoinConfig))
+    if is_worker_node:
+        # We'll only do this for worker nodes, control plane nodes are handled
+        # separately.
+        taints = src['bootstrap-node-taints'].strip().split()
+        if taints:
+            dest.extra_node_kubelet_args['--register-with-taints'] = ",".join(taints)