Add explicit noop gate job

This repo defines jobs in check queue, so we can remove the noop-jobs
template usage from project-config. To allow that, add a noop job to
gate queue.

Change-Id: I828e0f257ce82f059c15f49757ce0b832ca73448

.zuul.yaml

@@ -6,6 +6,9 @@
             voting: false
         - sysinv-tox-unittest:
             voting: false
+    gate:
+      jobs:
+        - noop
 
 - job:
     name: sysinv-tox-unittest