Provide ability to add content to configuration.py

The configuration.py file is managed via a template: https://github.com/anderssh/ash-netbox/blob/master/templates/configuration.py.epp

To enable LDAP authentication, this line is required:
```
REMOTE_AUTH_BACKEND = 'netbox.authentication.LDAPBackend'
```
With the current template structure, there is no way to add content like this.