Use templates for cover and lower-constraints

Use openstack-tox-cover template, this runs the cover job
in the check queue only.

Use openstack-lower-constraints-jobs template

Remove jobs that are part of the templates.

Sort list of templates alphabetically.

Change-Id: Ibaca28e6653c41856648dae5411d335d801dfeb8

.zuul.yaml

@@ -1,17 +1,10 @@
 - project:
     templates:
+      - check-requirements
+      - openstack-cover-jobs
+      - openstack-lower-constraints-jobs
+      - openstack-pypy-jobs-nonvoting
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
-      - check-requirements
-      - openstack-pypy-jobs-nonvoting
       - publish-openstack-docs-pti
-    check:
-      jobs:
-        - openstack-tox-lower-constraints
-    gate:
-      jobs:
-        - openstack-tox-lower-constraints
-    post:
-      jobs:
-        - openstack-tox-cover