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/variables.tf

@@ -1,35 +1,20 @@
 ## Global ##
 
-variable "prefix" {
-  default = ""
-}
+# Required variables
 
 variable "machines" {
   description = "Cluster machines"
   type = map(object({
     node_type = string
     ip        = string
+    netmask   = string
   }))
 }
 
-variable "inventory_file" {
-  default = "inventory.ini"
-}
-
-variable "network" {
-  default = "VM Network"
-}
+variable "network" {}
 
 variable "gateway" {}
 
-variable "dns_primary" {
-  default = "8.8.4.4"
-}
-
-variable "dns_secondary" {
-  default = "8.8.8.8"
-}
-
 variable "vsphere_datacenter" {}
 
 variable "vsphere_compute_cluster" {}
@@ -44,6 +29,35 @@ variable "vsphere_server" {}
 
 variable "vsphere_hostname" {}
 
+variable "ssh_public_keys" {
+  description = "List of public SSH keys which are injected into the VMs."
+  type        = list(string)
+}
+
+variable "template_name" {}
+
+# Optional variables (ones where reasonable defaults exist)
+
+variable "folder" {
+  default = ""
+}
+
+variable "prefix" {
+  default = "k8s"
+}
+
+variable "inventory_file" {
+  default = "inventory.ini"
+}
+
+variable "dns_primary" {
+  default = "8.8.4.4"
+}
+
+variable "dns_secondary" {
+  default = "8.8.8.8"
+}
+
 variable "firmware" {
   default = "bios"
 }
@@ -52,15 +66,6 @@ variable "hardware_version" {
   default = "15"
 }
 
-variable "template_name" {
-  default = "ubuntu-focal-20.04-cloudimg"
-}
-
-variable "ssh_public_keys" {
-  description = "List of public SSH keys which are injected into the VMs."
-  type        = list(string)
-}
-
 ## Master ##
 
 variable "master_cores" {