Use extracted contents of release tarball as artifacts

[mxmxchere]

An important step for the (initial) publishing of a new target-asset-set (maybe I will find a etter name for it soon) is to detect the artifacts that have been created by the builder. The current approach traverses the feature-tree and includes all artifacts that are created by convert.* scripts. This approach does not detect all relevant artifacts (logs for example are missing). According to @5kt the contents of the tarballs in the end of the build process should be a pretty good fit (except for the nested artifacts for PXE for example, but this is going to be addressed in another issue).

I will try not to duplicate any code, unless possible. So the challenge is to find an elegant drop-in point to reuse as much code as possible.

To detect and push release assets of a build the current command line call is as follows

glcli oci push --container localhost:5000/test1 --architecture amd64 --cname kvm_dev --version 1443.9 --gardenlinux_root /home/muench/vcs/git/gardenlinux

My idea is to have a new command push-tarball which is intended to be called by the CI the command above can then still be used when running locally on a dev-station.

glcli oci push-tarball --container localhost:5000/test1 --tarball  ali-gardener_prod-amd64-1592.1-ec945aa9.tar.xz 

This approach will rely on the filename to read arch, cname and version and will detect the artifacts by unpacking the tarball and push each extracted file as its own layer.

[mxmxchere]

I think valuable steps to test next are

• push all release tarballs of a release (to detect if all file-types are known yet)
• push some release tarballs in parallel to see if we have a race condition when signing the index