Use templates for cover and lower-constraints

Small cleanups:

* Use openstack-tox-cover template, this runs the cover job
  in the check queue only. Remove individual cover jobs.
* Use openstack-lower-constraints-jobs template, remove individual
  jobs.
* Sort list of templates

Change-Id: I2c04ce37615837707b14f895a52743e3c5fbc9ee

zuul.d/projects.yaml

@@ -1,18 +1,11 @@
 - project:
     templates:
-      - openstack-python-jobs
-      - publish-openstack-docs-pti
       - check-requirements
+      - openstack-cover-jobs
+      - openstack-lower-constraints-jobs
+      - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
       - openstackclient-plugin-jobs
+      - publish-openstack-docs-pti
       - release-notes-jobs-python3
-    check:
-      jobs:
-        - openstack-tox-lower-constraints
-    gate:
-      jobs:
-        - openstack-tox-lower-constraints
-    post:
-      jobs:
-        - openstack-tox-cover