Remove redundant default=None for config options

The cfg module sets the default values as None by default. There
is no need to set this again.

The default is set as None:
http://docs.openstack.org/developer/oslo.config/opts.html#oslo.config.cfg.Opt

Change-Id: I5ab948b1571e51d05f6eb9f35720d0eecc3a56ab
Merge "Remove redundant default=None for config options"