Custom amount of NAT gateways

[aiell0]

Describe the solution you'd like.

Right now there are only two options for the amount of NAT gateways I want - either one or the amount of availability zones I am using. That's a bit limiting, us-east-1 for example has 6 availability zones and that can be overkill for some organizations who are cost conscious. What if I only want 3?

Describe alternatives you've considered.

I usually just disallow the module to create NAT gateways and create my own if I want a custom amount, and I felt that it should be in the module. The addition of the feature seemed simple enough.

Additional context

None

[bryantbiggs]

us-east-1 for example has 6 availability zones and that can be overkill for some organizations who are cost conscious. What if I only want 3?

How do you setup routes for this scenario? what if someone wants 2 NAT gateways, how would you setup routes for 6 subnets to 2 NAT gateways - and how do you decide which AZ/subnets to place the NAT gateways within?

[aiell0]

@bryantbiggs IMO the best approach would be to let the user configure which AZs/subnets they wanted the NAT gateways in (including having multiple in the same subnet if they wanted for whatever reason). That part could be configurable.

As far as deciding how to route private subnets to them, I think we would have to balance them within the module. For example, if you wanted 2 NAT gateways in us-east-1 and you are deploying in all 6 AZs overall, then have us-east-1a, us-east-1c, and us-east-1e route to one NAT and us-east-1b, us-east-1d, and us-east-1f route to the other one.

I took a more naive approach in my PR but if the above sounds good, I will modify it to go with the above. Let me know how that sounds!