Add X-Forwarded-For into log_formats defaults

[mpolenchuk]

In order to log the X-Forwarded-For client ip address or the real
client ip address add LogFormats default directive.

[HelenCampbell]

Hey @mpolenchuk , thank you for your contribution. However before I merge this all tests would need to have passed, and this change would also require a documentation update.

[bmjen]

@mpolenchuk @HelenCampbell The pcci-trusty test failure can be ignored, I've already fixed the fastcgi problem in master. I'll merge as everything else looks ok.

[antaflos]

@mpolenchuk shouldn't that be %>s instead of just %s? Both common and combined use %>s so forwarded should follow suit, no?

[mpolenchuk]

@antaflos in terms of sameness yes. I've overlooked it. Thank you.