Change Jenkins to use a single consistent data-binding field.

See also: https://github.com/stapler/stapler/pull/39

This commit includes several changes to accommodate the aforementioned change to Stapler (moving away from 'stapler-class'), as well as to make Jenkins consistently use this same JSON field (moving away from 'kind').


NOTE: Stapler will continue to accept 'stapler-class' for backwards compatibility, and similarly this will continue to pass 'stapler-class' for plugins that have keyed off of it.
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