- Bump version: 9.11.13 → 9.11.14

- Cleanup SLE12 boot image descriptions

- Bump version: 9.11.12 → 9.11.13

- Cleanup SLE boot image descriptions

Don't use packages which does not exist on SLE, Fixes #523

- Improve coding style to make flake8 happy

This commit includes the exact Exception class that the 'except' statement
catches. According to the new flake8 3.5.0 version it should be explicit
rather than implicit.

- Add OCI reference in skopeo copy call and umoci config call

Skopeo, since v1.24, does no longer assume 'latest' as the default
tag/reference and requires explicit tag or reference in skopeo