virtual_network_resource_group: use vnet peering

[mcgov]

Updates subnet addressPrefix selection and naming conventions. The goal is to allow an existing vnet to share a single address space with test VMs, while allowing the test VMs to avoid conflicting address spaces for each RG. This deprecates the need for the subnet_prefix parameter in runbooks; subnets are now named based on the address space (minus the mask).

• name subnets after the addressPrefix, always create subnets based on address spaces and resource group index.
• always share 10.0.0.0/24, other nics have address space prefix of 10.$rg_index.$subnet_index.0/24
• swap vnet address space to 10.0.0.0/8 to fit the new larger address space
• remove subnet_prefix and references to it in documentation and code.