varnish: do not mark X-analytics header on tier-2 on bits

Bits varnish works differently from the other ones, as it does not have
the backend-frontend distinction, so we need to add the php=XXX
X-Analytics bit only on the tier-1 servers.

Change-Id: I3e64211a59fe43e1e53145fbe6bc164240dc3718
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>