Use openstack-tox-cover template

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

Remove jobs and use template instead.

Change-Id: I998c864f6d63921d6b424f81b0d38561edf5675c

.zuul.yaml

@@ -1,12 +1,8 @@
 - project:
     templates:
+      - openstack-cover-jobs
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
       - release-notes-jobs-python3
       - publish-openstack-docs-pti
-    check:
-      jobs:
-        - openstack-tox-cover:
-            voting: false
-