vnet module - add option to re-use subnets

[FoSix]

Describe the solution you'd like

At this moment the vnet module works in 2 modes:

1. it creates a VNET
2. it can use an existing VNET to create subnets, NSGs and UDRs.

This could be extended to 3^rd options, where this module would re-use both: an existing VNET and existing subnets. And create only NSGs, UDRs and then assign them to proper subnets.

Describe alternatives you've considered

Additional context