Use template for lower-constraints

Small cleanups:

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

Change-Id: Ibafdc46bff0bb37c709e7ccc8bdf205447eebeef
Needed-By: https://review.openstack.org/623229

.zuul.yaml

@@ -37,22 +37,21 @@
 
 - project:
     templates:
-      - os-client-config-tox-tips
-      - shade-functional-tips
-      - shade-tox-tips
       - check-requirements
-      - publish-openstack-docs-pti
+      - lib-forward-testing
+      - lib-forward-testing-python3
+      - openstack-lower-constraints-jobs
       - openstack-python-jobs
       - openstack-python36-jobs
       - openstack-python37-jobs
-      - lib-forward-testing
-      - lib-forward-testing-python3
+      - os-client-config-tox-tips
+      - publish-openstack-docs-pti
       - release-notes-jobs-python3
+      - shade-functional-tips
+      - shade-tox-tips
     check:
       jobs:
         - osc-functional-devstack-tips
-        - openstack-tox-lower-constraints
     gate:
       jobs:
         - osc-functional-devstack-tips
-        - openstack-tox-lower-constraints