Add enough testing on stackviz gate

While dropping py2 support, we dropped py3.5 support also
- Ic14a02c879bcef08885d831ebbe25083ec464a86

which broke the stable jobs using xenial node which does not
have py3.6.

This is fixing that and bring the py3.5 support back
- I0e6d57a49643d588b1329a0c8f1dd7afc71d5bdf

To avoid such issue in future, let's test the all users of
it which is nothing but integration jobs along with stable
branch jobs.

Change-Id: I7118fd5cfdeff7419f2477b64334767d13a41155

.zuul.yaml

@@ -10,7 +10,25 @@
     templates:
       - nodejs8-jobs
       - publish-openstack-docs-pti
+      - openstack-python35-jobs
       - openstack-python3-ussuri-jobs
+    check:
+      jobs:
+      - tempest-full-py3:
+          irrelevant-files: &irrelevant-files
+            - ^.*\.rst$
+            - ^doc/.*$
+            - ^releasenotes/.*$
+            - ^/stackviztests/.*$
+            - ^.coveragerc$
+            - ^.gitignore$
+            - ^.gitreview$
+      - tempest-full-train-py3:
+          irrelevant-files: *irrelevant-files
+      - tempest-full-stein-py3:
+          irrelevant-files: *irrelevant-files
+      - tempest-full-rocky-py3:
+          irrelevant-files: *irrelevant-files
     periodic:
       jobs:
         - periodic-package-stackviz-element