Use template for lower-constraints

Small cleanups:

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

Change-Id: Id6aab3caa21ff06c5150e0bce21d4ab6b015bfe8
Needed-By: https://review.openstack.org/623229

.zuul.yaml

@@ -1,16 +1,15 @@
 - project:
     templates:
+      - check-requirements
+      - openstack-lower-constraints-jobs
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
-      - check-requirements
-      - release-notes-jobs-python3
       - publish-openstack-docs-pti
+      - release-notes-jobs-python3
     check:
       jobs:
-        - openstack-tox-lower-constraints
         - horizon-openstack-tox-python3-django111
     gate:
       jobs:
-        - openstack-tox-lower-constraints
         - horizon-openstack-tox-python3-django111