Use openstack-tox-cover template

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

Remove jobs and use template instead.

Change-Id: I347ba793764073f46731a8953030976188e1a1c4

.zuul.yaml

@@ -1,5 +1,6 @@
 - project:
     templates:
+      - openstack-cover-jobs
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
@@ -9,10 +10,8 @@
     check:
       jobs:
         - legacy-tricircle-dsvm-functional
-        - openstack-tox-cover
         - legacy-tricircle-dsvm-multiregion
     gate:
       jobs:
         - legacy-tricircle-dsvm-functional
-        - openstack-tox-cover
         - legacy-tricircle-dsvm-multiregion