Use template for lower-constraints

Small cleanups:

* Use openstack-lower-constraints-jobs template, remove individual
  jobs.
* Sort list of templates

Change-Id: I65a08fe2b6dbed7c0c0ba3ae8750ef330fbd4cf3
Needed-By: https://review.openstack.org/623229

.zuul.yaml

@@ -1,15 +1,10 @@
 - project:
     templates:
+      - check-requirements
+      - openstack-lower-constraints-jobs
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
-      - publish-openstack-docs-pti
-      - check-requirements
       - openstackclient-plugin-jobs
+      - publish-openstack-docs-pti
       - release-notes-jobs-python3
-    check:
-      jobs:
-        - openstack-tox-lower-constraints
-    gate:
-      jobs:
-        - openstack-tox-lower-constraints