Merge "Add missing release note for Glance multi store move."

releasenotes/notes/glance_default_store-39e6725ba8a47db7.yaml

+---
+upgrade:
+  - |
+    Glance deployment now uses Multi-Store support. Users that have
+    ``default_stores`` in their service config overrides for
+    ``glance-api.conf`` should remove it and use ``default_backend`` if needed.