Log outgoing X-Analytics header for nginx

Nginx still logged X-CS as analytics header, although X-Analytics is
now the correct analytics header. So we switch to logging
X-Analytics. In addition to that, we switch to logging the header of
the response instead of the request, as the varnish and the effective
backend mangle the header. This is in concordance with varnish, which
also logs the response's X-Analytics header.

Change-Id: Id7b35850af38cc84babc5e4e7ab883c23d5e0383