Docs/Reno: Conditional nova UID/GID

Adding documentation and a release note for the conditional
nova UID/GID work done in the IRR role in:

  https://review.openstack.org/#/c/290110/

Closes-Bug: 1544889
Depends-On: I881555a896455ce3c897d56a143a8a25c088880b
Change-Id: I98a904beb99d9b0bebd35d49c275a2e2f4c59279

doc/source/install-guide/configure-nova.rst

@@ -105,6 +105,28 @@ multipath support in nova through a configuration override:
       libvirt:
           iscsi_use_multipath: true
 
+Shared storage and synchronized UID/GID
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Deployers can specify a custom UID for the nova user and GID for the nova group
+to ensure they are identical on each host. This is helpful when using shared
+storage on compute nodes because it allows instances to migrate without
+filesystem ownership failures.
+
+By default, Ansible will create the nova user and group without specifying the
+UID or GID. To specify custom values for the UID/GID, set the following
+Ansible variables:
+
+.. code-block:: yaml
+
+    nova_system_user_uid = <specify a UID>
+    nova_system_group_gid = <specify a GID>
+
+**WARNING:** Setting this value **after** deploying an environment with
+OpenStack-Ansible can cause failures, errors, and general instability. These
+values should only be set once **before** deploying an OpenStack environment
+and then never changed.
+
 --------------
 
 .. include:: navigation.txt

releasenotes/notes/conditional-nova-uid-gid-f75c1faa0c3da575.yaml

@@ -0,0 +1,4 @@
+---
+features:
+  - Deployers can optionally set a UID and/or GID for the nova user and group.
+    This is helpful for environments with shared storage.