Add support for devince binding

In some cases we need to use block devices directly (Physical volume
storage).

This new variable allows the user to mount devices with the --device
option for flatcar baremetal.

How to use :

add a list of objects like this to the cluster definition :

worker_bind_devices = [
  { source = "/dev/sdb" target = "/dev/sdb" },
  { source = "/dev/sdc" target = "/dev/sdd" },
  { source = "/dev/sdd" target = "/dev/sdd" }
]

This will bind the source device on the target directory in evry node.

bare-metal/flatcar-linux/kubernetes/worker/butane/worker.yaml

@@ -77,6 +77,9 @@ systemd:
           --register-with-taints=${taint} \
           %{~ endfor ~}
           --node-labels=node.kubernetes.io/node
+          %{~ for device in compact(split(",", node_devices)) ~}
+          --device=${device.source}:${device.target}
+          %{~ endfor ~}
         ExecStart=docker logs -f kubelet
         ExecStop=docker stop kubelet
         ExecStopPost=docker rm kubelet