Add external_medium? to Repository data model

Kiwi- and Autoyast export both need this check so it does make sense
to add it to the data model.
Use external_medium? method from data model
Filter external media from Autoyast export
Add SLES12 DVD to repository factory example
Merge pull request #480 from SUSE/filter_dvd_repositories_from_autoyast

Filter dvd repositories from autoyast