Use template for lower-constraints

Small cleanups:

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

Change-Id: I63bfcd9bc21011b446fd1c54cb64c5568c601687
Needed-By: https://review.openstack.org/623229

.zuul.yaml

@@ -1,11 +1,12 @@
 - project:
     templates:
+      - check-requirements
+      - openstack-cover-jobs
+      - openstack-lower-constraints-jobs
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
-      - openstack-cover-jobs
       - publish-openstack-docs-pti
-      - check-requirements
       - release-notes-jobs-python3
     check:
       jobs:
@@ -17,12 +18,10 @@
         - watcherclient-tempest-functional
         - watcher-tempest-zone_migration
         - watcher-tls-test
-        - openstack-tox-lower-constraints
     gate:
       queue: watcher
       jobs:
         - watcher-tempest-functional
-        - openstack-tox-lower-constraints
 
 - job:
     name: watcher-tempest-dummy_optim