docs: Improve 'AggregateImagePropertiesIsolation' docs

When we introduced the 'ImageMetaProps' o.vo in Liberty, we lost the
ability to consume arbitrary metadata configured for images. This
affects users of the 'AggregateImagePropertiesIsolation' filter, who may
have set such arbitrary metadata and expected their instances to be
restricted to host aggregates matching that metadata.

The world has changed a lot since Liberty was released, and it's
probably too late and maybe even a little unwise to let that genie back
out of its bottle, however, we can and should probably do a better job
of warning people of this change in behavior in our documentation. Do
just this.

Change-Id: If7245a90711bd2ea13095ba26b9bc82ea3e17202
Signed-off-by: Stephen Finucane <[email protected]>
Related-Bug: #1741810

Author: stephenfin

doc/source/admin/configuration/schedulers.rst

@@ -163,6 +163,14 @@ The following sections describe the available compute filters.
 AggregateImagePropertiesIsolation
 ---------------------------------
 
+.. versionchanged:: 12.0.0 (Liberty)
+
+    Prior to 12.0.0 Liberty, it was possible to specify and use arbitrary
+    metadata with this filter. Starting in Liberty, nova only parses
+    :glance-doc:`standard metadata <admin/useful-image-properties.html>`. If
+    you wish to use arbitrary metadata, consider using the
+    :ref:`AggregateInstanceExtraSpecsFilter` filter instead.
+
 Matches properties defined in an image's metadata against those of aggregates
 to determine host matches: