Standard API looks like this:

```python
def create_volume(self, size, name, location=None, snapshot=None):
```

If a user would use regular and not keyword arguments, this method would not work as expected.

To make it compatible with the base API, please change the signature to:

```python
def create_volume(self, size, name, location=None, snapshot=None, image=None, use_existing=True):
```