[horizon] Transition Ussuri to EM

This transition the ussuri 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: Ib4077666d86a5cfe8c1dd55e010e01e599ace9ac

deliverables/ussuri/horizon.yaml

@@ -50,6 +50,10 @@ releases:
     projects:
       - repo: openstack/horizon
         hash: ab2c91efca0613bd67f2e74af06654114e0a4448
+  - version: ussuri-em
+    projects:
+      - repo: openstack/horizon
+        hash: ab2c91efca0613bd67f2e74af06654114e0a4448
 branches:
   - name: stable/ussuri
     location: 18.3.0