Define a specific Zuul tenant for Zuul project

This change starts the process of moving the Zuul project into its own
tenant in Zuul. Today we are only adding zuul/project-config and
opendev/base-jobs, but eventually we'll move other zuul projects into
here (after we confirm base jobs are working correctly).

Change-Id: I9ea4e5a65bd4a213bdd1360250b12871d71dfe59
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

zuul/main.yaml

@@ -1580,3 +1580,17 @@
               - plugins/reviewnotes
               - plugins/singleusergroup
               - plugins/verify-status
+
+- tenant:
+    name: zuul
+    max-nodes-per-job: 10
+    source:
+      gerrit:
+        config-projects:
+          - zuul/project-config
+          # Only use jobs and secrets from this repo, we do not want
+          # the project definition.
+          - opendev/base-jobs:
+              include:
+                - job
+                - secret