Use templates lower-constraints, remove cover

Use openstack-lower-constraints-jobs template

Remove jobs that are part of the templates.

Remove cover job, this one is not working at all and always fails. If
anybody wants to enable it again, they can add the openstack-cover-jobs
template and fix it - I couldn't find an easy way to fix it without
knowledge of this repo.

Change-Id: I3b50c8384507d22339ed11f556dbe32181fe0518

.zuul.yaml

@@ -1,5 +1,6 @@
 - project:
     templates:
+      - openstack-lower-constraints-jobs
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
@@ -10,10 +11,3 @@
       jobs:
         - watcherclient-tempest-functional:
             voting: false
-        - openstack-tox-lower-constraints
-        - openstack-tox-cover:
-            voting: false
-
-    gate:
-      jobs:
-        - openstack-tox-lower-constraints