Gate on v3 devstack job

The devstack base job in in use in many projects, but it is not being
gated here in devstack. Let's add it to the list so that we don't
accidentally break it.

Change-Id: Iea13235a8438d4b540f9f27b94aed13e719481dc
diff --git a/.zuul.yaml b/.zuul.yaml
index b9ffb34..7c53d45 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -97,3 +97,6 @@
       jobs:
         - devstack
         - devstack-multinode
+    gate:
+      jobs:
+        - devstack