tf apply fails if bastion is not enabled

[hhoover]

I think the bastion module needs a count to check var.bastion_host_enabled

│ Error: Invalid index
│ 
│   on .terraform/modules/vpc/main.tf line 138, in module "bastion":
│  138:   key_name                    = aws_key_pair.bastion[0].key_name
│     ├────────────────
│     │ aws_key_pair.bastion is empty tuple
│ 
│ The given key does not identify an element in this collection value: the collection has no elements.