Adding support for updating Availability Zones v2

[johncowie]

Extends #141 with the following:

• If a list of availability zones is provided, then adds a hardcoded offset to the inferred private_subnet_cidr, so there's a decent gap between the public and private subnet cidrs. This means that if a new availability zone is added to the list, the existing private subnets won't need to get destroyed to make way for the new public subset.
• Also supports explicitly specifying the public and private subnet cidrs with the availabilty zones, in which case these will be favoured instead of the calculated ones. If users are adding a new AZ this will allow them to ensure that the subnets of previously created AZs will be preserved.