dynamicproxy: Pass through existing XFF data too

This changes the XFF configuration of nginx to pass through the existing
XFF data through the proxy, so if there are multiple levels of proxies
then the web application behind these can use the data.

This is needed for the account-creation-assistance project on Labs, as
sometimes we see people come through proxies which are on the TrustedXFF
list, and it is beneficial to us to be able to get the same IP address
that Wikipedia itself will then use.

This is completely untested - I don't know nginx and I got my
information from http://www.networkinghowtos.com/howto/set-the-x-forwarded-for-header-on-a-nginx-reverse-proxy-setup/

Change-Id: Ib27796b464998f6f384dff155a60f712c3ad7d94