Change bare-metal workers variable to optional

* To accompany the restructure of the bare-metal modules to
allow discrete workers to be defined and attached to a cluster
(#1295), the `workers` variable (older way, used for defining
homogeneous workers inline) should be optional and default
to an empty list
* Add docs covering inline vs discrete metal workers

Fix #1301

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

@@ -52,6 +52,7 @@ List of worker machine details (unique name, identifying MAC address, FQDN)
   { name = "node3", mac = "52:54:00:c3:61:77", domain = "node3.example.com"}
 ]
 EOD
+  default     = []
 }
 
 variable "snippets" {