Add terraform-kubernetes-installer version / commit as a Terraform output variable

[jlamillan]

In order to maintain some relationship between a cluster and the version of terraform-kubernetes-installer it was deployed with, we should be exporting the version / commit id as an output variable in outputs.tf as follows:

output "terraform-kubernetes-installer-version" {
  value = "$ git rev-parse HEAD"
}

That way, they can always sync to terraform-kubernetes-installer-version and have a clean terraform plan.