This is consistent with the previous simplication of
build targets in the opendev environment to refer to
"opensuse15" being the alias of "latest stable openSUSE Leap 15.x".
Change-Id: I904a3ca0d6dbddd2bb1a673836ab6a0ad249526d
Use openSUSE 15.1 as default, which is the latest released stable
openSUSE release.
Remove leftovers for unmaintained openSUSE 42.2 images.
Depends-On: https://review.opendev.org/#/c/660126/
Change-Id: I0b204b7b3d7ae74b6749320b3bfe1ca89d154ebb