Let it use debian bullseye and rename the directory once, so we don't need to rename it in the future when switching the debian version again.
The reason I'm doing this now is that building with debian 9 failed due to obs.osmocom.org related changes. It would be trivial to fix that, but I decided to rather upgrade to the latest debian version.
release-tarball-build-dist: move osmo-ci code here
Add a jenkins.sh that does the same as the previous lengthy shell command in the jenkins-job-builder file, and add osmocom-release-tarballs.sh. This makes it easier to test the script, and only patching one repository is needed from now on when changing to a newer debian version.
