Consider changing vnet specification

[JimMadge]

:strawberry: Suggested change

From https://github.com/alan-turing-institute/data-safe-haven/pull/1772#discussion_r1567342991

Vnet address ranges are predetermined by functions. The infrastructure may not be resilient to changes in the network structure. Applying a new network structure to an existing deployment may introduce problems.

We should check,

• Changes to the network structure are propagated correctly
  • Are one set of rules used consistently everywhere?
• Do we need to be strict about ad-hoc changes?

[JimMadge]

Related to #1900. If that is closed we will be able to have a simpler definition of vnet ranges.