Add Zuul config
Add Zuul config

This adds a branch-aware job ("gerrit-build") to build Gerrit.  The
version of this job that runs on this branch will have the specified
submodule repos checked out as well.  Other branches will have their
own versions.  The bulk of the job is inherited from gerrit-build-base
which is defined in the zuul/jobs repository.

Depends-On: https://gerrit-review.googlesource.com/255652
Change-Id: I95cbd50433746014057bddd38256f1692ec1f801

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