Add build-openstack-python-tarball job

So that folks can gate on their tarballs working properly.

Also make sure tox is around in pre-run not run.

Change-Id: Id167cdaa2c527637c35a68e86f936b77b27ca983

playbooks/python-tarball/pre.yaml

@@ -7,3 +7,4 @@
       bindep_command: /usr/bindep-env/bin/bindep
       bindep_fallback: /usr/local/jenkins/common_data/bindep-fallback.txt
     - revoke-sudo
+    - ensure-tox

playbooks/python-tarball/run.yaml

@@ -1,8 +1,4 @@
 - hosts: all
-  pre_tasks:
-    - name: Ensure tox is installed
-      shell: type tox || pip install --user tox
-
   roles:
     - role: tox
       tox_extra_args: -vv python setup.py sdist

zuul.yaml

@@ -36,6 +36,14 @@
     run: playbooks/python-tarball/run
     post-run: playbooks/python-tarball/post
 
+- job:
+    name: build-openstack-python-tarball
+    parent: python-sdist
+    description: |
+      Build a tarball using tox but do not upload it anywhere.
+    pre-run: playbooks/python-tarball/pre
+    run: playbooks/python-tarball/run
+
 - project:
     name: openstack-infra/openstack-zuul-jobs
     check: