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.

If new submodules are added, we should update this job definition to
add their repos here.  When a new stable branch is created from the
contemporary master branch, it will already contain the correct
configuration for the job on the new stable branch.

Depends-On: https://gerrit-review.googlesource.com/255652
Change-Id: I9aa8156a1a33f3d8659d4ac9f9634807ce1a8e41

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