Use template for lower-constraints

Small cleanups:

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

Change-Id: I65ea0d049586364af48ea9ea1ce91f244473b3fc
Needed-By: https://review.openstack.org/623229

.zuul.yaml

@@ -1,17 +1,17 @@
 - project:
     templates:
+      - check-requirements
+      - openstack-lower-constraints-jobs
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
       - publish-openstack-docs-pti
-      - check-requirements
       - release-notes-jobs-python3
     check:
       jobs:
         - freezer-ubuntu-xenial
         - freezer-centos-7:
             voting: false
-        - openstack-tox-lower-constraints
         - openstack-tox-pylint
         - freezer-tempest-agent
         - freezer-tempest-scheduler
@@ -19,7 +19,6 @@
       queue: freezer
       jobs:
         - freezer-ubuntu-xenial
-        - openstack-tox-lower-constraints
         - openstack-tox-pylint
         - freezer-tempest-agent
         - freezer-tempest-scheduler