Remove default=None when set value in Config
Remove default=None when set value in Config

By default oslo.cfg sets the default values as None [1], there is no need
to explicitly do this.

[1] https://github.com/openstack/oslo.config/blob/master/oslo_config/cfg.py#L829

Change-Id: I97c70439c76fd17712f79371dd0ebc0ab12caabf
Closes-bug: #1323975

openstack/castellan,openstack/castellan