terraform: use volumes instead of ephemeral local disks on STACKIT

Ephemeral local disks are discouraged on STACKIT.
Use volumes instead.
This sets an upper bound of 5GB on the boot disk.
If Constellation OS images ever grow beyond that, we will run into
problems.

terraform/infrastructure/openstack/modules/instance_group/main.tf

@@ -58,7 +58,8 @@ resource "openstack_compute_instance_v2" "instance_group_member" {
   block_device {
     uuid                  = var.image_id
     source_type           = "image"
-    destination_type      = "local"
+    destination_type      = "volume"
+    volume_size           = "5"
     boot_index            = 0
     delete_on_termination = true
   }