Use templates for cover and lower-constraints

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

Use openstack-lower-constraints-jobs template

Remove jobs that are part of the templates.

Change-Id: I58f3bc27aab2885514b8c6a8379e1c2214bd1afd

.zuul.yaml

@@ -1,5 +1,7 @@
 - project:
     templates:
+      - openstack-cover-jobs
+      - openstack-lower-constraints-jobs
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
@@ -8,12 +10,3 @@
       - lib-forward-testing
       - lib-forward-testing-python3
       - release-notes-jobs-python3
-    check:
-      jobs:
-        - openstack-tox-lower-constraints
-    gate:
-      jobs:
-        - openstack-tox-lower-constraints
-    post:
-      jobs:
-        - openstack-tox-cover