Stop using Jenkins SCM plug-in

This depends on slave script support and should not be approved
until I86c77695d8c374c94932664026ab7a541686ae74 has a chance to
appear in all current nodepool images.

* macros.yaml: Add a branch-git-prep builder which passes a BRANCH
environment variable into the gerrit-git-prep.sh slave script.

* python-bitrot-jobs.yaml: Pass the {branch} parameter to the
branch-git-prep builder to specify the branch on which to run stable
bitrot jobs, instead of using the SCM plug-in.

* translation-jobs.yaml
* zuul.yaml: Pass master as the branch for the branch-git-prep
builder instead of using the SCM plug-in.

Change-Id: Ied8b4c111172b654cc90938221690651a5b458df
Merge "Stop using Jenkins SCM plug-in"