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: I02c0c5e8b706c1170bd5b782eb624b4429d97053

.zuul.yaml

@@ -2,6 +2,7 @@
     templates:
       - nodejs4-jobs
       - publish-openstack-docs-pti
+      - openstack-cover-jobs
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
@@ -17,9 +18,6 @@
             vars:
               node_version: 6
             voting: false
-        - openstack-tox-cover:
-            voting: false
-
     gate:
       jobs:
         - refstack-tox-py27-func-mysql