Deprecate allow_additional_image_properties

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. So, Deprecate the allow_additional_image_properties in Ussuri scheduled for removal in the V development cycle, consistent with the standard OpenStack deprecation policy.