Add release note regarding nova upgrade

There are steps that operator need to do if they have existing config
files.

Change-Id: Ie523db44a903bb50652d3cbaac111475cce7cc8b

releasenotes/notes/nova-cellsv2-76cbf7b746968dae.yaml

+---
+prelude: >
+  - Kolla now deploys Nova in cells v2 mode.
+upgrade:
+  - |
+    There is new required option in passwords.yaml -
+    placement_keystone_password. To populate this variable with random
+    password, simply add it to passwords.yaml and re-run kolla-genpwd
+  - |
+    There is new required group in inventory. Please add these lines to your
+    inventory:
+
+    [placement:children]
+    control
+
+    [placement-api:children]
+    placement