Move nginx cache into /srv/pbench

[dbutenhof]

PBENCH-1316

Our deployed containerized server maps /var/lib (the default NGINX cache location) to /home, which has only 26Gb free. Instead, configure NGINX to use to our large Pbench volume at /srv/pbench/nginx in order to be able to transfer larger datasets.

[dbutenhof]

changing the Nginx configuration to use a directory different from the default.

That's what I said. But I get you're stuck on the word "map". I guess I can squash it different.