Use template for lower-constraints

Small cleanups:

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

Change-Id: Idb31ca14478641cba6f896af35fa766d7bdb9e1e
Needed-By: https://review.openstack.org/623229

.zuul.yaml

@@ -34,26 +34,25 @@
 
 - project:
     templates:
+      - check-requirements
+      - lib-forward-testing
+      - openstack-lower-constraints-jobs
+      - openstack-pypy-jobs-nonvoting
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
       - publish-openstack-docs-pti
-      - check-requirements
-      - openstack-pypy-jobs-nonvoting
-      - lib-forward-testing
       - release-notes-jobs-python3
     check:
       jobs:
         - swiftclient-swift-functional
         - swiftclient-functional
         - swiftclient-functional-py2
-        - openstack-tox-lower-constraints
     gate:
       jobs:
         - swiftclient-swift-functional
         - swiftclient-functional
         - swiftclient-functional-py2
-        - openstack-tox-lower-constraints
     post:
       jobs:
         - openstack-tox-cover