Use template for lower-constraints

Small cleanups:

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

Change-Id: Ie5bec468aa6a8a8c437fd3196e1eeea039374f00
Needed-By: https://review.openstack.org/623229

.zuul.yaml

@@ -1,13 +1,8 @@
 - project:
     templates:
       - check-requirements
+      - openstack-lower-constraints-jobs
       - openstack-python-jobs-neutron
       - openstack-python35-jobs-neutron
       - openstack-python36-jobs-neutron
       - periodic-stable-jobs-neutron
-    check:
-      jobs:
-        - openstack-tox-lower-constraints
-    gate:
-      jobs:
-        - openstack-tox-lower-constraints