pybind/ceph_volume_client: Optionally authorize existing auth-ids

pybind/ceph_volume_client: Optionally authorize existing auth-ids

Optionally allow authorizing auth-ids not created by ceph_volume_client
via the option 'allow_existing_id'. This can help existing deployers
of manila to disallow/allow authorization of pre-created auth IDs
via a manila driver config that sets 'allow_existing_id' to False/True.

Fixes: https://tracker.ceph.com/issues/48555
Signed-off-by: Kotresh HR <khiremat@redhat.com>