Merge "Deprecate allow_additional_image_properties"

specs/ussuri/approved/glance/deprecate-allow-custom-props.rst

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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+======================================================
+Spec Lite: Deprecate allow_additional_image_properties
+======================================================
+
+:project: glance
+
+:problem: The ``allow_additional_image_properties`` configuration option
+          was originally introduced to prevent users from flooding the
+          image_properties table with junk.  Given that we now have the
+          ``image_property_quota`` option,
+          ``allow_additional_image_properties`` is unnecessary.  Further,
+          if an operator were to set it False, all sorts of stuff would
+          break as multiple OpenStack services (for example, Cinder, Nova)
+          currently write/rely on custom image properties.  Of course, in
+          such a case, an operator would have to change the option to
+          True to get everything working again; but the point is that
+          if this setting is always True, then it is unnecessary and
+          polluting the code with unnecessary branch points.
+
+:solution: Deprecate the ``allow_additional_image_properties`` in Ussuri
+           scheduled for removal in the V development cycle, consistent with
+           the standard OpenStack deprecation policy.
+
+:impacts: None.  Operators who really want to use this option can instead
+          set ``image_property_quota`` to 0.  ('0' means zero; a negative
+          value means 'unlimited' for this option.)
+
+:assignee: rosmaita (or anyone who would like to address this low-hanging
+           fruit)