Add the nova-next job to the experimental queue

The nova-next job is being moved from openstack-zuul-jobs
to the nova repo and so we'll remove it's usage from
project-config, therefore we need to define it's usage for
devstack here.

The related project-config change is:

I36d96f89b3e5323746fcbcef5cc7e4d0384a184d

Depends-On: I24a5f73c29094a23e2fdef8ee8b43601300af593
Change-Id: I28971dc7e5cb0b5cf9698e5251a7bb099e63f3db
diff --git a/.zuul.yaml b/.zuul.yaml
index 8bc5103..a90df2f 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -212,3 +212,6 @@
       jobs:
         - devstack
         - devstack-unit-tests
+    experimental:
+      jobs:
+        - nova-next