Minor release 9.1.0 for Mistral (for stable/train)

* There's been a number of useful bugfixes and performance
  improvements backported to stable/train. It's all worth releasing
  a new minor version.

Change-Id: I058888ffb003fd4cb2636e4da4e78f7ef4e8d8a9

deliverables/train/mistral.yaml

@@ -47,6 +47,10 @@ releases:
         hash: c40fc775f4912cad335120a565f85d4260c965f4
       - repo: openstack/mistral-dashboard
         hash: 82df9c6400eb1c5e74c14df16439fcfb5491ddfd
+  - projects:
+      - hash: 30039178714ea776df2b60a0f49427c239eb6d22
+        repo: openstack/mistral
+    version: 9.1.0
 branches:
   - name: stable/train
     location: 9.0.0.0rc1