There is currently no testing of the Leap 15 images in OSH.
This addresses it by:
- Using the values_overrides folder according to the multi-os
spec, creating value override files there for changes that
needs to happen on Leap 15 images.
- Point to the right images using the previously created folder,
to allow using those in CI easily.
- Change CI to use previously created overrides.
Depends-On: https://review.openstack.org/#/c/651501
Change-Id: I520d3676195c62b253a19397c86b0d0fbabee710
a828d38316