Use template for lower-constraints

Small cleanups:

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

Change-Id: Ia8c8464a332118bfc7ec09938dbe6154c902aa6e
Needed-By: https://review.openstack.org/623229

.zuul.d/project.yaml

@@ -1,10 +1,5 @@
 ---
 - project:
     templates:
+      - openstack-lower-constraints-jobs
       - openstack-python36-jobs
-    check:
-      jobs:
-        - openstack-tox-lower-constraints
-    gate:
-      jobs:
-        - openstack-tox-lower-constraints