remove default=None for config options

In the cfg module default=None is set as the default value, there are a 
lot of StrOpts but few with default=None, to be more unify and clear, we 
should remove it.

Change-Id: I2a5dab763fe9cc07e6c1dbed4628e9cf2ca28bcd
Closes-Bug: #1323975
Merge "remove default=None for config options"