Issue #2613336 by stBorchert, webflo: Convert migration_group to a third party setting

config/schema/migrate_plus.schema.yml

...example/config/install/migrate.migration.beer_comment.yml

...te_example/config/install/migrate.migration.beer_node.yml

...te_example/config/install/migrate.migration.beer_term.yml

...te_example/config/install/migrate.migration.beer_user.yml

..._advanced/config/install/migrate.migration.wine_terms.yml

...entity_form_display.node.migrate_example_wine.default.yml

...entity_view_display.node.migrate_example_wine.default.yml

....entity_view_display.node.migrate_example_wine.teaser.yml

....migrate_example_wine.field_migrate_example_adv_image.yml

...ll/field.storage.node.field_migrate_example_adv_image.yml

migrate_plus.module

src/Tests/MigrationGroupTest.php