Use template for lower-constraints

Small cleanups:

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

Change-Id: Ie4fa0a49318e20b9c4a32db44de0bdb59666eddd
Needed-By: https://review.openstack.org/623229

.zuul.yaml

@@ -1,17 +1,12 @@
 - project:
-    check:
-      jobs:
-        - openstack-tox-lower-constraints
-    gate:
-      jobs:
-        - openstack-tox-lower-constraints
     templates:
-      - openstack-python-jobs
-      - openstack-python35-jobs
       - check-requirements
       - lib-forward-testing
       - lib-forward-testing-python3
+      - openstack-lower-constraints-jobs
+      - openstack-python-jobs
+      - openstack-python35-jobs
+      - openstack-python36-jobs
+      - periodic-stable-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
-      - periodic-stable-jobs
-      - openstack-python36-jobs