Use template for lower-constraints

Small cleanups:

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

Change-Id: I3912c425d65b1c7b4730c25b19e4b0f592f93441
Needed-By: https://review.openstack.org/623229

.zuul.yaml

@@ -1,28 +1,25 @@
 - project:
     templates:
+      - check-requirements
+      - openstack-lower-constraints-jobs
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
-      - check-requirements
       - release-notes-jobs-python3
     check:
       jobs:
         - freezer-api-ubuntu-xenial
         - freezer-api-centos-7:
             voting: false
-        - openstack-tox-lower-constraints
         - openstack-tox-pylint
         - freezer-tempest-basic
         - freezer-tempest-basic-sqlalchemy
-
     gate:
       jobs:
         - freezer-api-ubuntu-xenial
-        - openstack-tox-lower-constraints
         - openstack-tox-pylint
         - freezer-tempest-basic
         - freezer-tempest-basic-sqlalchemy
-
     experimental:
       jobs:
         - freezer-api-opensuse-423