Tool generates schemas with invalid references

If I run the tool against Istio OpenAPI schemas, it generates a schema successfully, but the schema contains invalid references. E.g. I run it like this:

```
openapi2jsonschema --kubernetes --strict --expanded local/path/to/schema.json
```

It generates schema with references like this:

```json
{
...snip...
"$ref": "istio.networking.v1beta1.TLSRoute.json"
...snip...
}
```

And yet, the tool does not actually generate this file ``istio.networking.v1beta1.TLSRoute.json``. So loading of the schema using ``kubeval`` fails outright.