Move the default version from Product into TestPlan
Move the default version from Product into TestPlan

Placing the default product version inside the Product class was
not a good idea. This commit removes the version attribute from
the Product class. TestPlan class has been adjusted accordingly by
adding a new attribute 'version' as this is the proper place for
this information.

ShaolongHu/python-nitrate,thrix/python-nitrate,psss/python-nitrate