terraform: Prepare the workstation tools

This allows to enhance the default terraform install tool with a proxmox
plugin. Allowing to manage the proxmox infrastructure within text files.

Related T1785
Related T1720

proxmox/terraform/prepare-workstation.md

+This is the required tooling for the following to work.
+
+# terraform-provider-proxmox
+
+go module to install
+
+```
+git clone https://github.com/Telmate/terraform-provider-proxmox
+cd terraform-provider-proxmox
+
+# compile terraform proxmox provider
+export GOPATH=`pwd`
+make setup
+make
+make install
+
+# Install so that terrafor actually sees the plugin
+mkdir -p ~/.terraform.d/plugins/linux_amd64
+cp -v ./bin/* ~/.terraform.d/plugins/linux_amd64/
+```
+
+At the end of this, `terraform init` within /proxmox/terraform/ should now
+work.
+
+Doc: https://github.com/Telmate/terraform-provider-proxmox/blob/master/README.md
+

[Phabricator Migration user]

Some references in the commit message have been migrated:

• T1785 is now infra/sysadm-environment#1785
• T1720 is now infra/sysadm-environment#1720