Switch to a Python API to write nginx confs

New Python API to write nginx confs (instead of templates) in dploi_fabric/toolbox/nginx_dsl.py

Now API to postprocess nginx confs (via aforementioned Python API):

```python

def somefunc(conf):
     pass

{
    '<STAGE>': {
        'postprocess_nginx_conf': somefunc
    }
}```