This change removes some redundant definitions of default values
according to the following three points.
- The get method returns None by default if the key doesn't exist,
so it is unnecessary to define None explicitly.
- Setting '' as parameter defaults is redundant because we always use
"if CONF.get('<name>'):" to check whether the parameter is set.
- In oslo.config all options default to None unless the default
parameter is explicitly set. Thus defining default=None is redundant.
Change-Id: Ib14b24b9054fe4d0efc3e03f9c6683ae999e3202
ee1990ef65