[oslo] 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: I79d4e545e3d49399c4c657694be6089cf06bbbbd

deliverables/train/oslo.service.yaml

@@ -23,6 +23,10 @@ releases:
     projects:
       - repo: openstack/oslo.service
         hash: a7621c8c46378fb456f306dcf5f8cc7836b110d2
+  - version: train-em
+    projects:
+      - repo: openstack/oslo.service
+        hash: a7621c8c46378fb456f306dcf5f8cc7836b110d2
 branches:
   - name: stable/train
     location: 1.40.2