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: Id210b2f15dcba9dcf9ad75e0436995e7d5c1b62b

.zuul.yaml

@@ -192,6 +192,8 @@
     templates:
       - openstackclient-plugin-jobs
       - osc-tox-unit-tips
+      - openstack-cover-jobs
+      - openstack-lower-constraints-jobs
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
@@ -213,11 +215,6 @@
             # The functional-tips job only tests the latest and shouldn't be run
             # on the stable branches
             branches: ^(?!stable)
-        - openstack-tox-lower-constraints
     gate:
       jobs:
         - osc-functional-devstack
-        - openstack-tox-lower-constraints
-    post:
-      jobs:
-        - openstack-tox-cover