Only Scale CPU and Ram not Storage

[JakobStadlhuber]

There is an option in hetzner when scaling servers to allow downscaling by not scaling the storage, it would be amazing if this would be possible too.

Example with a possible thew param scale_storage

    {
      name        = "agent-small-x",
      server_type = "cax21",
      location    = "fsn1",
      scale_storage = false
      count       = 1,
      labels      = [],
      taints      = [],
      placement_group = "agent-group-fsn1-x1"  # Ensure this is unique
    }

[WebSpider]

I guess your use-case here is:

1. Define a nodepool, and have it spawned by terraform
2. Change the server_type in the nodepool
3. Now you want terraform to run by just replacing the VM-type, with the option to keep the disksize original (to support downsizing, or just to save storage costs with Hetzner)

Is that correct?

If so, there already is support in the hcloud terraform provider for this, the option is called keep_disk, so adding this should be relatively straightforward.

[WebSpider]

Make a small PR for this, let's see if I got it right the first time :D

#1347