3.8: use core definition import property

[michaelklishin]

This image still relies on the legacy management.load_definitions configuration key instead of a newer load_definitions.

These may seem identical but there are some differences:

• The latter option does not require the management plugin to be enabled
• The latter option is built into the node boot process and can happen e.g. after all plugins haven been activated.
  The is not the case for management.load_definition which, as any plugin initialization step, can happen only at plugin activation time.

The second point above means that any plugins that are enabled after the management plugin would not be available and their definitions would not pass validation and would be skipped.

Generating a config line that uses load_definitions instead of management.load_definitions should be all there is to do for this image. I'd be happy to contribute a PR after testing this change.

See rabbitmq/rabbitmq-server#2428 for the background.