Use template for lower-constraints

Small cleanups:

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

Change-Id: I50707932b24d0935f1cd04c334ff655e5d62a4fa
Needed-By: https://review.openstack.org/623229

.zuul.yaml

@@ -1,13 +1,8 @@
 - project:
     templates:
+      - check-requirements
+      - openstack-lower-constraints-jobs
       - openstack-python-jobs
       - openstack-python35-jobs-nonvoting
-      - check-requirements
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
-    check:
-      jobs:
-        - openstack-tox-lower-constraints
-    gate:
-      jobs:
-        - openstack-tox-lower-constraints