Add release note for redis in mistral

Mistral requires redis as of pike release,
notify users about this new feature and make sure
updrades enables redis.

Change-Id: I41e82869d036673181b4dba05ad07089691d0357

releasenotes/notes/mistral-redis-9f7da83c521056cc.yaml

+---
+features:
+  - |
+    Redis has been implemented as mistral coordination backend.
+upgrade:
+  - |
+    Redis is a required feature for mistral, please ensure redis
+    is enabled before upgrading mistral.