Allow the FAS url to be configurable.

Hm, in the config it looks like this
```python
config = dict(
    fas_credentials=dict(
        username='wat',
        password='wat',
        base_url='https://admin.stg.fedoraproject.org/accounts',
    )
)
```

So, while it wouldn't hurt to rename ``base_url`` to ``fas_base_url``, it would be kind of redundant.