Transit gateway support

[drewmullen]

Closes: #40, #39

• optionally build TGW subnets
• must supply transit_gateway_id
• optionally create route_table_association to subnets, must supply transit_gateway_route_table_id

[drewmullen]

updates have been requested:

• route from public/private to tgw
• allow tgw to route to nat

  subnets = {
    public = {
      netmask = 24
      # "0.0.0.0/0" is explicitly blocked for map to tgw
      route_to_transit_gateway = ["10.0.0.0/8"]
    }

    private = {
      netmask = 24
      # if route_to_nat = true
      # route_to_transit_gateway != 0.0.0.0/0
      route_to_nat = true
      route_to_transit_gateway = ["0.0.0.0/0"]
    }

    transit_gateway = {
      netmask            = 24
      transit_gateway_id = aws_ec2_transit_gateway.example.id
      route_to_nat       = true # default false
      transit_gateway_default_route_table_association = true
      transit_gateway_default_route_table_propagation = true
    }
  }

[drewmullen]

updates have been requested:

• route from public/private to tgw
• allow tgw to route to nat

  subnets = {
    public = {
      netmask = 24
      # "0.0.0.0/0" is explicitly blocked for map to tgw
      route_to_transit_gateway = ["10.0.0.0/8"]
    }

    private = {
      netmask = 24
      # if route_to_nat = true
      # route_to_transit_gateway != 0.0.0.0/0
      route_to_nat = true
      route_to_transit_gateway = ["0.0.0.0/0"]
    }

    transit_gateway = {
      netmask            = 24
      transit_gateway_id = aws_ec2_transit_gateway.example.id
      route_to_nat       = true # default false
      transit_gateway_default_route_table_association = true
      transit_gateway_default_route_table_propagation = true
    }
  }

moving to a separate issue: https://github.com/aws-ia/terraform-aws-vpc/issues/41