Always prefer DVD medium to download (not only during update) (bnc#865819)

- 3.1.5
Merge pull request #38 from yast/prefer_CD

Always prefer DVD medium to download (not only during update) (bnc#865819)
fix iso type detection(BNC#852436)
small rubify
release
Merge pull request #39 from yast/fix_iso_detect

Fix iso detect