Allow XFF spoofing from the trusted IPs

In order to do automated testing of the varnish+zero configurations,
allow test frameworks to spoof source IP so that varnish would treat
request as if comming from a Zero carrier.

Change-Id: I25e2b0bf01bac1f2739f90efa3725e18e4494a01
Merge "Allow XFF spoofing from the trusted IPs" into production