Read some config settings from env vars in paster make-config

It's a minor comment, but I'd be inclined to both a) make this bit easier to read and b) enforce the naming policy by doing something like:

```python
def _env(name, default):
   environment_variables['CKAN_' + name] = os.environ.get('CKAN_' + name, default)

...
_env('SMTP_SERVER', 'localhost')
_env('SMTP_STARTTLS', 'False')
...
```