default lookup doesn't retain value type

I am passing in an environment file with the following line in it:

```yaml
BaselineCapacity: 2
```

In my blueprint I have the following variable defined:
```python
"BaselineCapacity": {
"type": int,
"description": "The number of instances to maintain during market close hours"
},
```

And finally, in my config.yml I have the following under one of my stacks
```
BaselineCapacity: ${default BaselineCapacity::1}
```

When I run stacker build I get the following error:
```
Value for variable BaselineCapacity must be of type . Actual type: .
```