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: I80992c244a86029d003381d78918e308b08de735

.zuul.yaml

@@ -1,14 +1,17 @@
 - project:
     templates:
+      - check-requirements
+      - openstack-lower-constraints-jobs-horizon
       - openstack-python-jobs-horizon
       - openstack-python35-jobs-horizon
-      - check-requirements
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
     check:
       jobs:
         - openstack-tox-py36:
             voting: false
+            required-projects:
+              - openstack/horizon
         - nodejs-npm-run-lint:
             vars:
               node_version: 4
@@ -20,9 +23,6 @@
             required-projects:
               - openstack/horizon
         - horizon-openstack-tox-python3-django111
-        - openstack-tox-lower-constraints:
-            required-projects:
-              - openstack/horizon
     gate:
       jobs:
         - nodejs-npm-run-lint:
@@ -36,6 +36,3 @@
             required-projects:
               - openstack/horizon
         - horizon-openstack-tox-python3-django111
-        - openstack-tox-lower-constraints:
-            required-projects:
-              - openstack/horizon