Client prints 'None' when data is None , and client also prints
'None' when data is a string 'None'. But string 'None' is different
to None. To clarify the confusion, Client should print '-' instead of
'None' when data is None. Nova Client and Cinder Client has clarified
the confusion.
Change-Id: I6a8908fcb2df39d7f2c5fc4055cc41779d668d6c