Switch to openstack-python3-ussuri-jobs

Neutron was using train jobs template: openstack-python3-train-jobs
but now we are in Ussuri cycle so let's switch to new template.

Change-Id: I99d9a048cab9d101740d96aa422f6fe596357c26

.zuul.yaml

@@ -8,7 +8,7 @@
       - openstack-cover-jobs
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python3-train-jobs
+      - openstack-python3-ussuri-jobs
       - publish-openstack-docs-pti
       - periodic-stable-jobs
       - check-requirements