Allow BRANCH instead of ZUUL_REF in ggp

* modules/jenkins/files/slave_scripts/gerrit-git-prep.sh: Allow
callers to supply BRANCH in the environment for cases where there
will be no ZUUL_REF environment variable. This is mainly to support
use in periodic jobs, so that we can transition away from the
Jenkins SCM plug-in.

Change-Id: I86c77695d8c374c94932664026ab7a541686ae74
Merge "Allow BRANCH instead of ZUUL_REF in ggp"