windows: spit do_sysprep to reduce complexity

Add a new _exec_sysprep_tasks method that is called by do_sysprep and
hosts the actual code for executing the system preparation tasks
windows: Create a new registry module

This modules hosts the Registry class which contains all the registry
manipulation code previously hosted under the Windows class
Fix a bug in sysprep param handling in dialog_menu

If the needed sysprep parameters had all default values,
snf-image-creator would crash when trying to run the image
customization. This bug could not be triggered from the current os_type
classes.
Reset the password when configuring Windows

Don't ask the user to enter an Administrator password before cleaning up
a Windows image. Reset the password before starting the process and
assign the old value if needed at the end.