Deprecate os_glance_reserved

The os_glance_reserved option was added to support old releases without
namespace reservation. The feature was added in Wallaby[1] and all of
the current supported versions (Zed and later) all support the feature.

[1] https://docs.openstack.org/releasenotes/glance/wallaby.html#upgrade-notes

Change-Id: I626ac99cd8f206b202bce3e743e419af40248638

releasenotes/notes/deprecate-os_glance_reserved-bace16f21facca3b.yaml

@@ -0,0 +1,11 @@
+---
+upgrade:
+  - |
+    Default value of the ``[image-feature-enabled] os_glance_reserved`` has
+    been changed from ``False`` to ``True`` and now the reservation of
+    os_glance namespace is tested by default.
+
+deprecations:
+  - |
+    The ``[image-feature-enabled] os_glance_reserved`` option has been
+    deprecatd because glance reserves the os_glance namespace since Wallaby.

tempest/config.py

@@ -690,13 +690,12 @@ ImageFeaturesGroup = [
     cfg.BoolOpt('import_image',
                 default=False,
                 help="Is image import feature enabled"),
-    # NOTE(danms): Starting mid-Wallaby glance began enforcing the
-    # previously-informal requirement that os_glance_* properties are
-    # reserved for internal use. Thus, we can only run these checks
-    # if we know we are on a new enough glance.
     cfg.BoolOpt('os_glance_reserved',
-                default=False,
-                help="Should we check that os_glance namespace is reserved"),
+                default=True,
+                help="Should we check that os_glance namespace is reserved",
+                deprecated_for_removal=True,
+                deprecated_reason='os_glance namespace is always reserved '
+                                  'since Wallaby'),
     cfg.BoolOpt('manage_locations',
                 default=False,
                 help=('Is show_multiple_locations enabled in glance. '