docs: image schema customization restrictions

[Brian Rosmaita]

An operator can modify /etc/schema-image.json to include arbitrary properties (which was that file's original purpose) and assign them JSON types other than 'string'.

The type is enforced by image create/update but an end-user making a call that sets a value on one of these gets a 500. This is because everything in the image_properties table must be a string in the database. The API, however, won't accept a string value when the schema says it's boolean or some other non-string JSON data type). We should document:

(1) these things *must* be strings

(2) recommend that operators not delete items from the default schema-image.json file, because this will affect interoperability

(3) maybe recommend using metadefs instead of modifying the image schema?

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/717889

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/717889
Committed: https://git.openstack.org/cgit/openstack/glance/commit/?id=bd16ab256e6bf6b423f3bdbd366ba0b56d5bc619
Submitter: Zuul
Branch: master

commit bd16ab256e6bf6b423f3bdbd366ba0b56d5bc619
Author: khashf <email address hidden>
Date: Mon Apr 6 17:59:13 2020 -0700

    Add warning and note on image schema customization docs

    An operator can modify /etc/schema-image.json to include arbitrary
    properties (which was that file's original purpose) and assign them JSON
    types other than 'string'. The type is enforced by image create/update
    but an end-user making a call that sets a value on one of these gets a
    500.

    This patch add the following recommendations to the documentation:
    - Value of `type` of each property in the JSON scheme must be `string`
    - Do not delete items from the default schema-image.json file
    - If operator's need is more complicated, suggest them using metadefs

    Co-authored-by: Brian Rosmaita <email address hidden>

    Change-Id: I879f4440a14b1e8420e230de84bfba5e0419a4d4
    Closes-bug: #1856578