Define subnets address space about azuretre

[christoferlof]

To be split into two.

[marrobi]

Potentially useful library:

https://github.com/netaddr/netaddr: A
A network address manipulation library for Python

• iterating, slicing, sorting, summarizing and classifying IP networks
• supernetting and subnetting

[marrobi]

Also interesting: https://stackoverflow.com/questions/62780989/ipv4network-in-python-calculating-next-minimum-subnet-of-different-size

[ross-p-smith]

Quick comment. I don't think it is worth putting the CORE_ADDRESS_SPACE and TRE_ADDRESS_SPACE as defaults in Terraform as these are needed as ENV vars when you run the tests. Therefore, I think it will be problematic changing them in 2 places.