Add system-required project template

This gives us an empty check queue on every project so that zuul will
report syntax and config errors on push to all projects in the opendev
tenant.

Change-Id: Icaea32826bf408c3420e547b49b1f97b2a16dc9c

zuul.d/project-templates.yaml

@@ -0,0 +1,11 @@
+- project-template:
+    name: system-required
+    description: |
+      Jobs that *every* project in OpenDev CI should have by default.
+
+      This is automatically added to all projects in OpenDev CI, no
+      repository should use this directly.
+    # Include a check queue so that initially every repo has a check queue
+    # and we can report invalid zuul.yaml files.
+    check:
+      jobs: []

zuul.d/projects.yaml

@@ -1,3 +1,8 @@
+- project:
+    name: ^.*
+    templates:
+      - system-required
+
 - project:
     name: opendev/project-config
     check: