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.

Change-Id: Ie7971023eed04f6479c05c1d2a1dfd2a948b07e1

.zuul.yaml

@@ -1,20 +1,13 @@
 - project:
-    check:
-      jobs:
-        - openstack-tox-lower-constraints
-    gate:
-      jobs:
-        - openstack-tox-lower-constraints
     templates:
       - check-requirements
+      - lib-forward-testing
+      - lib-forward-testing-python3
+      - openstack-lower-constraints-jobs
+      - openstack-cover-jobs
       - openstack-python-jobs
       - openstack-python35-jobs
+      - openstack-python36-jobs
+      - periodic-stable-jobs
       - publish-openstack-docs-pti
-      - lib-forward-testing
-      - lib-forward-testing-python3
       - release-notes-jobs-python3
-      - periodic-stable-jobs
-      - openstack-python36-jobs
-    post:
-      jobs:
-        - openstack-tox-cover