[RFE] Terraform + Elemental cluster enhancements

[dustin-groh-dev]

Describe the solution you'd like

1. Provision an elemental cluster from assets in machine inventory within Rancher.
2. Allow custom pool sizing so that way elemental does not grab all machines in the inventory and only provisions the number of machines set on the pool size value in cluster management.
3. Have Terraform create the MachineInventorySelectorTemplates (related Ghost Node Pool in UI if cluster created outside of UI elemental-ui#220)
4. Make sure that TF can have the RKE2 version upgraded without breaking reconciliation in the rancher cluster.

[matttrach]

• create a rancher cluster that can provision elemental https://elemental.docs.rancher.com/
• generate machine inventory
• generate custom pool
• provision from pool
• attempt to overprovision (should fail)
• generate machine inventory selector templates
• provision an rke2 cluster from elemental machines
• upgrade rke2 cluster

[matttrach]

step one on this is a bit of a doozy, I will report back with progress as I make it