TrivialFix: Remove default=None when set value in Config
TrivialFix: 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: I3c06888d9d03525b521abd250123518f89a70649

openstack/smaug,openstack/smaug