Add utility module "if_then_list"

For use in modules that use a lot of conditionals + list variables.
This module is a little more convenient than performing the workarounds
needed to use conditionals with list values.

See: hashicorp/terraform#12453

if_then_list/outputs.tf

@@ -0,0 +1,17 @@
+output result {
+  /* value = "${null_resource.result.triggers.split}" */
+  value = "${local.result}"
+}
+
+locals {
+  then_string = "${join(var.separator, var.then)}"
+  else_string = "${join(var.separator, var.else)}"
+
+  result_string = "${
+    var.equality_condition_left == var.equality_condition_right
+      ? local.then_string
+      : local.else_string
+  }"
+
+  result = "${split(var.separator, local.result_string)}"
+}

if_then_list/terraform.tfvars-example

@@ -0,0 +1,4 @@
+equality_condition_left  = "equal"
+equality_condition_right = "noequal"
+then = ["then", "list"]
+else = ["else", "list", "else"]

if_then_list/variables.tf

@@ -0,0 +1,22 @@
+variable equality_condition_left {
+  description = ""
+}
+
+variable equality_condition_right {
+  description = ""
+}
+
+variable then {
+  description = ""
+  type        = "list"
+}
+
+variable else {
+  description = ""
+  type        = "list"
+}
+
+variable separator {
+  description = ""
+  default     = "^:--:^"
+}