Add release note for versionless keystone endpoint

Include an upgrade release note to make users know
that endpoint are going to be updated while upgrading
to Pike.

Change-Id: Iaa2bfce27e9d9cdbb574121147fc761810d7dd71

releasenotes/notes/keystone-versionless-endpoint-ae9274c81927d949.yaml

+---
+upgrade:
+  - Keystone version will be removed from keystone endpoints
+    while upgrading to Pike.