Use templates lower-constraints, update cover job

Use openstack-lower-constraints-jobs template.

Remove jobs that are part of the templates.

Remove post job, this is not checked by anybody. The current job is
setup as check job and compares to last change, so no need for a post
job - especially for a post job that nobody checks.

Change-Id: I4fefd5b29c1b16b886c72fc42fbedc07d82397c4

.zuul.yaml

@@ -1,5 +1,6 @@
 - project:
     templates:
+      - openstack-lower-constraints-jobs
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
@@ -13,7 +14,6 @@
             voting: false
         - sahara-tests-scenario
         - sahara-tests-tempest
-        - openstack-tox-lower-constraints
         - openstack-tox-cover:
             voting: false
         - sahara-grenade
@@ -22,7 +22,6 @@
       jobs:
         - sahara-tests-scenario
         - sahara-tests-tempest
-        - openstack-tox-lower-constraints
         - sahara-grenade
     experimental:
       jobs:
@@ -31,9 +30,6 @@
         - sahara-buildimages-mapr
         - sahara-tests-scenario-multinode-spark
         - sahara-tests-scenario-py3
-    post:
-      jobs:
-        - openstack-tox-cover
 
 - job:
     name: sahara-grenade