Add .zuul.yaml and python 3 Train unit tests

This commit adds .zuul.yaml and python 3 Train unit tests jobs which is
one of Train goals.

See the Train python3-updates goal document for details:
https://governance.openstack.org/tc/goals/train/python3-updates.html

Change-Id: If0e74b8939dc86924e4fea73ddb1e5e7495629fe

.zuul.yaml

@@ -0,0 +1,7 @@
+- project:
+    templates:
+      - openstack-cover-jobs
+      - openstack-python-jobs
+      - openstack-python3-train-jobs
+      - release-notes-jobs-python3
+      - publish-openstack-docs-pti