Cleanup .zuul.yaml
Cleanup .zuul.yaml

Some cleanups and improvements:
* Add required-projects to py36 job
* Use openstack-lower-constraints template instead of individual jobs
* Sort list of templates

Change-Id: I2292a28d588032ed2706b1314a7ccdad2579cb8a

openstack/magnum-ui,openstack/magnum-ui,openstack/magnum-ui,openstack/magnum-ui