[Hubs] Clarify address space is for private networking only

[flanakin]

⚠️ Problem

Original comment:

Hi all. This option stopped my deployment, as under no circumstances would I want this template deploying vnets etc.

The .8 behaviour is not very clear during the deployment as the prefix is mandatory whether you select public or private.

The only message being "Address space for the workload. A /26 is required for the workload." - which isn't quite true if you select public.

Originally posted by @psilantropy in #1183

🛠️ Solution

Clarify that the address space is only applicable for private networking. Make it hidden when public is specified, if possible.

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