Add VPC custom subnet blocks

[jonascrecencio]

Is your request related to a new offering from AWS?

• No 🛑

Is your request related to a problem? Please describe.

I need to create 3 subnets in us-east-1a to comunicate to my on-prem network. It does not need HA. To achieve that, I have to create 9 new subnets, distributed trough my var.azs = ["us-east-1a", "us-east-1b", "us-east-1c"].

The other 6 subnets will never be used. I need only the us-east-1a subnets.

Describe the solution you'd like.

Add customized private subnet blocks (like groups) with the possibility to define different AZ(s) and subnet_suffix (Name tags).

There are some standard blocks, like:

public_subnets: name-subnet-public-us-east-1a database_subnets: name-subnet-database-us-east-1a private_subnets: name-subnet-private-us-east-1a And so on...

They are distributed trough the var.azs.

However, It would be great if I could define aditional subnets that can be allocated in a specific AZ:

private_custom_blocks:

name-subnet-foo-us-west-1a name-subnet-bar-us-west-1a And so on...

Input a new optional variable to the module:

private_custom_blocks = {
  foo = {
    azs           = ["us-west-2a"]
    subnet_suffix = "subnet-foo"
    tags          = {}
    subnets       = ["10.10.192.0/27", "10.10.192.32/27", "10.10.192.64/27"]
  },
  bar = {
    azs           = ["us-west-2a", "us-west-2b", "us-west-2c"]
    subnet_suffix = "subnet-bar"
    tags          = {}
    subnets       = ["10.10.224.0/27", "10.10.224.32/27", "10.10.224.64/27"]
  },
  xpto = {
    azs           = ["us-west-2a", "us-west-2b", "us-west-2c"]
    subnet_suffix = "subnet-xpto"
    tags = {
      Foo = "bar"
    }
    subnets = ["10.10.240.0/27", "10.10.240.32/27", "10.10.240.64/27"]
  }
}

Describe alternatives you've considered.

1. Create a lot of subnets distributed trough var.azs that will never be alocated.
2. Create my own terraform module, but it wouldn't be available to community.
3. Use other blocks, like elasticache_subnets, to create the specific subnets, but it's not the right use for these vars.

Additional context

There is an initial PR describing the idea: #805