Use templates for cover and lower-constraints

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

Use openstack-lower-constraints-jobs template.

Remove jobs that are part of the templates.

Change-Id: I35cb7fc67c8dfab530bc59c526e3b89cb1a18488

.zuul.yaml

@@ -1,6 +1,8 @@
 - project:
     templates:
       - check-requirements
+      - openstack-cover-jobs-horizon
+      - openstack-lower-constraints-jobs-horizon
       - openstack-python-jobs-horizon
       - openstack-python35-jobs-horizon
       - publish-openstack-docs-pti
@@ -9,23 +11,15 @@
       jobs:
         - manila-ui-dsvm:
             voting: false
-        - openstack-tox-lower-constraints:
-            required-projects:
-              - openstack/horizon
         - horizon-openstack-tox-python3-django111
         - horizon-openstack-tox-py27dj19
         - horizon-openstack-tox-py27dj110
-        - openstack-tox-cover:
-            voting: false
     gate:
       jobs:
-        - openstack-tox-lower-constraints:
-            required-projects:
-              - openstack/horizon
         - horizon-openstack-tox-python3-django111
-
         - horizon-openstack-tox-py27dj19
         - horizon-openstack-tox-py27dj110
+
 - job:
     name: manila-ui-dsvm
     parent: legacy-dsvm-base