feature: expose `proxy_set_header Host` to be configurable

[chri2]

I added a reverse proxy to another site like this

short.my.doma.in (yunohost) → this.is.a.boring.long.domain.name.I.cant.remeber.net (somewhere else on the internet)

and it worked, but the external site didn't deliver the expected content, because the outgoings http Host header is set to short.my.doma.in and the wrong virtual host answers.

Adding proxy_set_header Host this.is.a.boring.long.domain.name.I.cant.remeber.net; to the nginx configuration did the trick.

It would be nice to be able to configure this through the apps settings.