Expose stub_status for telegraf plugin

Hi,

We currently don't seem to have metrics for no. of Nginx's active worker connections. This is exposed with the stub_status module[1] (active connections - accepts and handled). The charm should configure a vhost listening on localhost exposing this. We could then use something like the telegraf plugin fragment below[2]:

| [[inputs.nginx]]
| urls = ["http://localhost:6060/server_status"]

[1]http://nginx.org/en/docs/http/ngx_http_stub_status_module.html
[2]https://github.com/influxdata/telegraf/blob/release-1.14/plugins/inputs/nginx/README.md