autoupgrade support
small refactoring - share the common code

between the standard and Autoyast client
removed duplicate cache

available addons are already cached in Addon class
rename parameter enable_updates to disable_updates

to follow the logic
added tests (registration_ui_test.rb)
changes
improve code (code review fixes)
added FIXME note
Merge pull request #156 from yast/autoupgrade

Autoupgrade + small refactoring