Add an optional variable to specify a netnum offset

[robgonnella]

Describe the Feature

Allow users to specify an offset for the netnum value when calculating subnet cidr blocks. e.g.

subnet_names = ["one", "two"]
cidr_block = 10.10.0.0/16
netnum_offset = 1

Which would result in 2 subnets with cidr blocks, 10.10.16.0/20 and 10.10.32.0/20

Expected Behavior

Specifying a netnum offset would offset the call to cidrsubnet by the given value e.g.

cidrsubnet(var.cidr_block, ceil(log(var.max_subnets, 2)), count.index + var.netnum_offset)

Use Case

This valuable in cases where you want to use available addresses in a cidr block before needing to add an additional cidr block to the vpc.

Describe Ideal Solution

An optional variable (type = number) is added to the module that allows specification of a netnum offset. This offset is then used to offset the netnum value when calculating the subnet cidr block via the call to the cidrsubnet function.

Alternatives Considered

I could just add a new cidr block to the vpc, forego any netnum offset, and calling it a day. This is a totally valid and easy solution, but the addition of an offset doesn't seem too difficult either, and maybe someone else would also find value in this option.