Issue warning when undefined variables are accessed

All references to non existing variables will now generate a warning. Earlier puppet only warned about non existing qualified variables with more than one namespace segment. It is possible to disable these warnings by adding 'undefined_variables' to the setting 'disabled_warnings'. Note that in most cases there is no file/line information available, and this was one reason why only some cases were reported earlier. We expect to correct this in the next major version as it requires API breaking changes.