Enable project and volume usage to be optional, off by default, unless the use_project and use_volume variables are set to a positive number

resource.digitalocean_volume_attachment.this.tf

@@ -1,5 +1,5 @@
 resource "digitalocean_volume_attachment" "this" {
-  count      = var.volume_size > 0 ? 1 : 0
+  count      = var.use_volume > 0 ? 1 : 0
   droplet_id = digitalocean_droplet.this.id
   volume_id  = digitalocean_volume.this[0].id
 }