Use Proxmox UserData template for image deployment

This patch documents the require provisioning template for image-based
host deployment on Proxmox.
Users must select the "Proxmox UserData" provisioning template.

Refs "app/views/templates/provisioning/user_data/proxmox_user_data.erb"
in foreman_fog_proxmox

Author: maximiliankolb

guides/common/modules/proc_creating-hosts-on-proxmox.adoc

@@ -15,6 +15,9 @@ This causes DHCP conflicts with {ProjectServer} when booting new hosts.
 
 For image-based provisioning, use the pre-existing image as a basis for the new volume.
 
+.Prerequisites
+* Ensure that you have associated the *Proxmox UserData* provisioning template as *User data template* to your operating system entry.
+
 .Procedure
 include::snip_steps-create-a-host-tab-host.adoc[]
 . From the *Deploy on* list, select your Proxmox compute resource.