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

.zuul.yaml

@@ -0,0 +1,37 @@
+- job:
+    name: gerrit-base
+    parent: gerrit-setup
+    description: |
+      Base job for all Gerrit-related builds
+
+      This adds required projects needed for all Gerrit-related builds
+      (i.e., builds of Gerrit itself or plugins) on this branch.
+    required-projects:
+      - jgit
+
+- job:
+    name: gerrit-build
+    parent: gerrit-build-base
+    description: |
+      Build Gerrit
+
+      This builds Gerrit with the core plugins.
+    required-projects:
+      # This inherits from gerrit-base, so submodules listed above do
+      # not need to be repeated here.
+      - plugins/codemirror-editor
+      - plugins/commit-message-length-validator
+      - plugins/delete-project
+      - plugins/download-commands
+      - plugins/gitiles
+      - plugins/hooks
+      - plugins/plugin-manager
+      - plugins/replication
+      - plugins/reviewnotes
+      - plugins/singleusergroup
+      - plugins/webhooks
+
+- project:
+    check:
+      jobs:
+        - gerrit-build