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: I9c427fe8b76cf496e7aaf4c0451c7d8fe90e3bfb
Merge "Remove redundant default=None for config options"