This is much cleaner because the nginx role does not have to set the
fastcgi_cache variable to "false" by default, which was overridden by
host_vars/apollo.archlinux.org to "wiki", but the value was still
hardcoded in the config.

At first, I was wondering that the cache "zone" name should be
generalized to improve the configuration (from the original per-host to
per-service), but that would be an overkill since the fastcgi cache is
used only for the wiki...