Metadata conflicts due to each api/config API will refresh the metadata storage

[kevinzs2048]

Every time when api/config API access, it will always refresh the backend DB, which will induce metadata nonuniformity when multiple API access to rbd-target-api.
It's better to leave /api/config API only get config, do not frequently update the DB.

When we run the Cinder Ceph-iscsi tempest with multiple thread, I've noticed the metadata of tartget_config has been unexpectedly changed when multiple API access to update target, client, and together with api/config to concurrency refresh the backend metadata storage.

[kevinzs2048]

/assign