Use openSUSE 15.0 as default, which is the latest released stable
openSUSE release. Switch to https for accessing download.o.org
as encrypted transfers should be used by default.
Remove leftovers for definitely unmaintained openSUSE 13.x images
and split into old/new leap style versioning scheme for clarity.
Change-Id: Iab129eeee2b1a2563f0f0d2cb17bbad57c068e38
