ClientManager._region_name should be a public attribute

ClientManager._region_name is currently a private attribute, but downstream
projects like python-ironicclient need to access it when constructing service
clients (e.g. in the OSC plugin's make_client()).

Currently we access instance._region_name directly, which requires
# type: ignore[attr-defined] suppressions and relies on an implementation
detail.

Request: expose region_name as a public property or attribute on
ClientManager so downstream consumers don't need to reach into private
internals.

Reference: https://review.opendev.org/c/openstack/python-ironicclient/+/981668
Comment: https://review.opendev.org/c/openstack/python-ironicclient/+/981668/comment/f2fc7df3_69c5e600/