Use templates for cover

Small cleanups:

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

Change-Id: I4def2320a34e6ab152edaccd59e01a1a5d502680

zuul.d/layout.yaml

@@ -1,13 +1,10 @@
 - project:
     templates:
-      - tripleo-multinode-baremetal-minimal
-      - tripleo-multinode-container-minimal
       - check-requirements
+      - openstack-cover-jobs
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
-    check:
-      jobs:
-        - openstack-tox-cover:
-            voting: false
+      - tripleo-multinode-baremetal-minimal
+      - tripleo-multinode-container-minimal