When nested resource provider feature was added in Rocky,
root_provider_id column, which should be non-None value, is created in
the resource provider DB.
However, online data migration is only done implicitly via listing or
showing resource providers. With this patch, executing the cli command
`placement-manage db online_data_migrations`
makes sure all the resource providers are ready for nested provider
feature, that is, all the root_provider_ids in the DB have non-None
value.
Change-Id: I42a1afa69f379b095417f5eb106fe52ebff15017
Related-Bug:#1803925