Fixes various issues in vSphere Terraform code (#8178)

* Fixes various issues in vSphere Terraform code

Provided to address various shortcomings and to fix the following
issue in upstream Kubespray:

https://github.com/kubernetes-sigs/kubespray/issues/8176

* Resolves Terraform formatting issues

* Sets default prefix to human-readable name

* Documents new default prefix in README

contrib/terraform/vsphere/modules/kubernetes-cluster/variables.tf

@@ -5,7 +5,8 @@ variable "machines" {
   description = "Cluster machines"
   type = map(object({
     node_type = string
-    ip      = string
+    ip        = string
+    netmask   = string
   }))
 }