OpenShift 4 IPI installer failing with Terraform errors "No value for required variable" and "The root module input variable "xxx" is not set, and has no default"

Issue

• OpenShift 4 IPI installer failing with a series of Terraform No value for required variable errors, such as the following:

ERROR Error: No value for required variable
ERROR
ERROR   on variables-vsphere.tf line 5:
ERROR    5: variable "vsphere_url" {
ERROR
ERROR The root module input variable "vsphere_url" is not set, and has no default
ERROR value. Use a -var or -var-file command line argument to provide a value for
ERROR this variable.

• How to completely clean an environment of all the files created, downloaded or cached by previous RHOCP installation attempts ?