Evaluate var.create variable to set count to 1 or 0 (#62)

In order to avoid https://github.com/hashicorp/terraform/issues/18195

main.tf

@@ -2,7 +2,7 @@
 # Security group
 #################
 resource "aws_security_group" "this" {
-  count = "${var.create}"
+  count = "${var.create ? 1 : 0}"
 
   name        = "${var.name}"
   description = "${var.description}"