Use wsme.Unset as default value for API objects

wsme.Unset should be used instead of None as default value for API
objects as None is valid value that could be passed to constructor.

Closes-bug: #1385247
Change-Id: I94f6dd14a7befe217552cf2c6998bb1c1d2d338d
Merge "Use wsme.Unset as default value for API objects"