Add publish-openstack-python-branch-tarball post job

Start publishing branch-tarballs in the post pipeline.

Change-Id: I32144e8f2acc8f67b480ba276af10b84b75753e0
Signed-off-by: Paul Belanger <pabelanger@redhat.com>
diff --git a/.zuul.yaml b/.zuul.yaml
index 9bac031..271dd02 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -12,3 +12,6 @@
         - tox-docs
         - tox-pep8
         - tox-py35
+    post:
+      jobs:
+        - publish-openstack-python-branch-tarball