software/slaprunner: set Host header in nginx proxy

This host header is not passed by default, because we are already using
proxy_set_header, so we need to set it explicitly.

It seems that since we updated some packages in in 279486fe (stack/slapos:
version up some eggs with known vulnerabilties, 2021-01-28) slaprunner was
producing URLs with "localhost" as hostname when using redirect(url_for(...))