Added workaround for length() on a calculated value for efs_volume

see: hashicorp/terraform#10857

`length()` cannot be used on variables that aren’t known ahead of running terraform plan. To work around this, a `num_subnets` variable and output value has been added to the `aws_vpc` module, which is then used as to calculate the `count` attribute of `module.efs_volume.aws_efs_mount_target.mount_target`.

infra/terraform/modules/aws_vpc/main.tf

@@ -59,6 +59,10 @@ output "storage_subnet_ids" {
   value = ["${aws_subnet.storage.*.id}"]
 }
 
+output "storage_cidr_blocks" {
+  value = "${var.storage_cidr_blocks}"
+}
+
 output "dmz_subnets" {
   value = "${zipmap(aws_subnet.dmz.*.id, aws_subnet.dmz.*.availability_zone)}"
 }
@@ -103,4 +107,3 @@ output "nat_gateway_subnets" {
 # output "sg_inbound_http_id" {
 #   value = "${aws_security_group.inbound_http.id}"
 # }
-

infra/terraform/modules/efs_volume/efs.tf

@@ -24,5 +24,5 @@ resource "aws_efs_mount_target" "mount_target" {
   file_system_id  = "${aws_efs_file_system.fs.id}"
   subnet_id       = "${element(var.subnet_ids, count.index)}"
   security_groups = ["${aws_security_group.sg.id}"]
-  count           = "${length(var.subnet_ids)}"
+  count           = "${var.num_subnets}"
 }

infra/terraform/modules/efs_volume/inputs.tf

@@ -7,6 +7,8 @@ variable "subnet_ids" {
   type = "list"
 }
 
+variable "num_subnets" {}
+
 variable "performance_mode" {
   default = "generalPurpose"
 }

infra/terraform/platform/main.tf

@@ -103,6 +103,7 @@ module "airflow_storage_efs_volume" {
   vpc_id                 = "${module.aws_vpc.vpc_id}"
   node_security_group_id = "${module.aws_vpc.extra_node_sg_id}"
   subnet_ids             = "${module.aws_vpc.storage_subnet_ids}"
+  num_subnets            = "${length(module.aws_vpc.storage_cidr_blocks)}"
 }
 
 module "airflow_db" {