Update Zuul configuration
Update Zuul configuration

Couple of cleanups:
* Use openstack-lower-constraints-jobs template instead of
  individual jobs.
* The repo has both openstack-python36-jobs and extra cinder-tox-py36
  jobs, remove the cinder specific ones in benefit of the global one.
* Replace job cinder-tox-functional-py36 with
  openstack-tox-functional-py36.
* Separate job lines with empty line for better readability.
* Remove extra branches lines, those are wrong - each branch has its own
  .zuul.yaml file.

Change-Id: I6897cd0bb2386d31b8b17355cd88d9b42cf98fac

openstack/cinder,openstack/cinder,mahak/cinder,mahak/cinder