Restructure bare-metal module to use a worker submodule

* Add an internal `worker` module to the bare-metal module, to
allow individual bare-metal machines to be defined and joined
to an existing bare-metal cluster. This is similar to the "worker
pools" modules for adding sets of nodes to cloud (AWS, GCP, Azure)
clusters, but on metal, each piece of hardware is potentially
unique

New: Using the new `worker` module, a Kubernetes cluster can be defined
without any `workers` (i.e. just a control-plane). Use the `worker`
module to define each piece machine that should join the bare-metal
cluster and customize it in detail. This style is quite flexible and
suited for clusters with hardware that varies quite a bit.

```tf
module "mercury" {
  source = "git::https://github.com/poseidon/typhoon//bare-metal/flatcar-linux/kubernetes?ref=v1.26.2"

  # bare-metal
  cluster_name            = "mercury"
  matchbox_http_endpoint  = "http://matchbox.example.com"
  os_channel              = "flatcar-stable"
  os_version              = "2345.3.1"

  # configuration
  k8s_domain_name    = "node1.example.com"
  ssh_authorized_key = "ssh-rsa AAAAB3Nz..."

  # machines
  controllers = [{
    name   = "node1"
    mac    = "52:54:00:a1:9c:ae"
    domain = "node1.example.com"
  }]
}
```

```tf
module "mercury-node1" {
  source = "git::https://github.com/poseidon/typhoon//bare-metal/flatcar-linux/kubernetes/worker?ref=v1.26.2"

  cluster_name = "mercury"

  # bare-metal
  matchbox_http_endpoint  = "http://matchbox.example.com"
  os_channel              = "flatcar-stable"
  os_version              = "2345.3.1"

  # configuration
  name               = "node2"
  mac                = "52:54:00:b2:2f:86"
  domain             = "node2.example.com"
  kubeconfig         = module.mercury.kubeconfig
  ssh_authorized_key = "ssh-rsa AAAAB3Nz..."

  # optional
  snippets       = []
  node_labels    = []
  node_tains     = []
  install_disk   = "/dev/vda"
  cached_install = false
}
```

For clusters with fairly similar hardware, you may continue to
define `workers` directly within the cluster definition. This
reduces some repetition, but is not quite as flexible.

```tf
module "mercury" {
  source = "git::https://github.com/poseidon/typhoon//bare-metal/flatcar-linux/kubernetes?ref=v1.26.1"

  # bare-metal
  cluster_name            = "mercury"
  matchbox_http_endpoint  = "http://matchbox.example.com"
  os_channel              = "flatcar-stable"
  os_version              = "2345.3.1"

  # configuration
  k8s_domain_name    = "node1.example.com"
  ssh_authorized_key = "ssh-rsa AAAAB3Nz..."

  # machines
  controllers = [{
    name   = "node1"
    mac    = "52:54:00:a1:9c:ae"
    domain = "node1.example.com"
  }]
  workers = [
    {
      name   = "node2",
      mac    = "52:54:00:b2:2f:86"
      domain = "node2.example.com"
    },
    {
      name   = "node3",
      mac    = "52:54:00:c3:61:77"
      domain = "node3.example.com"
    }
  ]
}
```

Optional variables `snippets`, `worker_node_labels`, and
`worker_node_taints` are still defined as a map from machine name
to a list of snippets, labels, or taints respectively to allow some
degree of per-machine customization. However, fields like
`install_disk`, `kernel_args`, `cached_install` and future options
will not be designed this way. Instead, if your machines vary it
is recommended to use the new `worker` module to define each node

bare-metal/flatcar-linux/kubernetes/ssh.tf

@@ -16,7 +16,6 @@ resource "null_resource" "copy-controller-secrets" {
   depends_on = [
     matchbox_group.install,
     matchbox_group.controller,
-    matchbox_group.worker,
     module.bootstrap,
   ]
 
@@ -45,37 +44,6 @@ resource "null_resource" "copy-controller-secrets" {
   }
 }
 
-# Secure copy kubeconfig to all workers. Activates kubelet.service
-resource "null_resource" "copy-worker-secrets" {
-  count = length(var.workers)
-
-  # Without depends_on, remote-exec could start and wait for machines before
-  # matchbox groups are written, causing a deadlock.
-  depends_on = [
-    matchbox_group.install,
-    matchbox_group.controller,
-    matchbox_group.worker,
-  ]
-
-  connection {
-    type    = "ssh"
-    host    = var.workers.*.domain[count.index]
-    user    = "core"
-    timeout = "60m"
-  }
-
-  provisioner "file" {
-    content     = module.bootstrap.kubeconfig-kubelet
-    destination = "/home/core/kubeconfig"
-  }
-
-  provisioner "remote-exec" {
-    inline = [
-      "sudo mv /home/core/kubeconfig /etc/kubernetes/kubeconfig",
-    ]
-  }
-}
-
 # Connect to a controller to perform one-time cluster bootstrap.
 resource "null_resource" "bootstrap" {
   # Without depends_on, this remote-exec may start before the kubeconfig copy.
@@ -83,7 +51,6 @@ resource "null_resource" "bootstrap" {
   # while no Kubelets are running.
   depends_on = [
     null_resource.copy-controller-secrets,
-    null_resource.copy-worker-secrets,
   ]
 
   connection {
@@ -99,4 +66,3 @@ resource "null_resource" "bootstrap" {
     ]
   }
 }
-