this is again token driver specific and thus should not be accessed directly from the token structure.
I think we need to add a method to the ```TokenDriver``` interface, something like 
```python
def credential(self, token)
``` 
which is intended to extract and return the credential from the said token.

we can then impl a concrete version of it in ```JumpgateTokenDriver``` something like:
```python
def credential(self, token):
    return token.get('api_key')
```