subnet: enable availability_zones

• 2.8.1 -- 2023-04-10
  • subnets:
    • enable availability_zones, to be able to select less than the full set of available zones (say, for do2-gov, where we had 2 but now they have
      3 zones).
    • fix up the cidr math
    • this drops a new variables.subnets.tf for the examples, to include the optional availability_zones
    • as we are nearing completion with updating all of the vpc code to TF 1.x, this should not be a problem, and it really only applies in do2-gov