Zuul parameter ZUUL_URL

Zuul 2.0 passes to jobs an additional parameter ZUUL_URL which instructs
workers from where to clone repositories.

This is NOT the same as the zuul-url defined by Jenkins Job Builder
which is the notification endpoint set on the jobs and is unrelated.

Change-Id: I4924a5b7ae99571efd70eeb188ddd3d821a4c61b
Merge "Zuul parameter ZUUL_URL"