* Fix IP pools of subnets to take into account the 'offset' when searching
if a pool contains an IP address and when converting from value to index
and from index to value.
* Fix the way subnets are created from 'logic/networks'
* Rename 'validate_network_params' function to 'validate_subnet_params'
  and move it from 'networks' to 'subnets' packages.