Fix servers log cache issue

Summary:
On infra.nasqueron.org, a browser refresh was needed to update the log,
ctrl + shift + r for example. Yet, headers are configured not to cache it.

If headers are specified in both location and if block, the more precise
block wins (here the if):

"There could be several add_header directives. These directives are
inherited from the previous configuration level if and only if there
are no add_header directives defined on the current level."

We then need a more precise configuration with every headers.

Reference: http://nginx.org/en/docs/http/ngx_http_headers_module.html

Test Plan: Tested on Ysul for https://api.nasqueron.org/servers-log/all.json

Reviewers: DorianWinty

Reviewed By: DorianWinty

Differential Revision: https://devcentral.nasqueron.org/D2770