Only run tempest-full on the master branch

Further work needs to be done to backport devstack changes to
stable branches.

Change-Id: Ia1ffaa951e2e58a45aaff3bf75526185a6923a4f
diff --git a/.zuul.yaml b/.zuul.yaml
index 0588bc9..10788d7 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -16,6 +16,8 @@
 - job:
     name: tempest-full
     parent: devstack-tempest
+    # This currently only works on the master branch.
+    branches: master
     description: |
       Base integration test with Neutron networking and py27.
       Former names for this job where: