[feature] Enable nginx_status

[cbeaujoin-stellar]

In order to monitor nginx, it would be great to be able to enable Nginx status module in the site-conf.j2 template.

    {% if openwisp2_nginx_status%}
    location = /nginx_status {
        stub_status;
        allow 127.0.0.1;    # only allow requests from localhost
        deny all;           # deny all other hosts
        }
    {% endif %}

Enable with (false by default):

openwisp2_nginx_status: true

[nemesifier]

Wouldn't sending a request to /static/staticfiles.json allow you to obtain the same goal without the need of these extra lines of code?

[cbeaujoin-stellar]

It may provide same information but is less seamless to integrate with metricbeats nginx module: https://www.elastic.co/docs/reference/beats/metricbeat/metricbeat-module-nginx