[horizon] Transition Train to EM

This transition the train branch to extended maintenance.
Changes for bugfixes and things the team deems important are
still encouraged, but there will no longer be official releases
off of the branch.

Please +1 if the team is ready for us to proceed with this
transition, or -1 if there are any final backports currently in
flight that we should wait for. For the latter case, please
update the patch with the new commit hash after doing a final
release to get those changes out so we know to proceed with the
transition.

Change-Id: I5c1e5a27cc0ea3f51857d545d804df240e797381

deliverables/train/horizon.yaml

@@ -41,6 +41,10 @@ releases:
     projects:
       - repo: openstack/horizon
         hash: b72b7e7ac92bd8ce246e9539d2442d729dec4cde
+  - version: train-em
+    projects:
+      - repo: openstack/horizon
+        hash: b72b7e7ac92bd8ce246e9539d2442d729dec4cde
 repository-settings:
   openstack/horizon: {}
 cycle-highlights: