Accept initial worker node labels and taints map on bare-metal

[dghubble]

• Add worker_node_labels map from node name to a list of initial node label strings
• Add worker_node_taints map from node name to a list of initial node taint strings
• Unlike cloud platforms, bare-metal node labels and taints are defined via a map from node name to list of labels/taints. Bare-metal clusters may have heterogeneous hardware so per node labels and taints are accepted
• Only worker node names are allowed. Workloads are not scheduled on controller nodes so altering their labels/taints isn't suitable

module "mercury" {
  ...

  worker_node_labels = {
    "node2" = ["role=special"]
  }

  worker_node_taints = {
    "node2" = ["role=special:NoSchedule"]
  }
}

Related: #429