Add sphinx_python to the reno jobs

We need to set the version of python we use for the reno jobs so
that its able to run the pre-setup tasks. This issue is being
worked on upstream [0][1], however, until those changes are
merged we'll need to carry these vars.

[0] https://review.opendev.org/#/c/668954
[1] https://review.opendev.org/#/c/669008

Change-Id: Idd4edf2173dd62112e27ec2501b3f8ccb4c4427e
Signed-off-by: Kevin Carter <kecarter@redhat.com>

zuul.d/layout.yaml

@@ -4,7 +4,9 @@
     - tripleo-ansible-molecule-jobs
     check:
       jobs:
-      - build-reno-releasenotes
+      - build-reno-releasenotes:
+          vars:
+            sphinx_python: python3
       - openstack-tox-linters
       - tripleo-ansible-docs
       - tripleo-ci-centos-7-scenario004-standalone:
@@ -30,7 +32,9 @@
           irrelevant-files: *irrelevant_scenario_files
     gate:
       jobs:
-      - build-reno-releasenotes
+      - build-reno-releasenotes:
+          vars:
+            sphinx_python: python3
       - openstack-tox-linters
       - tripleo-ansible-docs
       - tripleo-ci-centos-7-scenario004-standalone: