Changes

OBS: cleanup: new script

Add a script for cleaning old sources uploaded to OBS. This was needed
as we were running out of space on the OBS server, due to source
tarballs being generated in nightly jobs (e.g. Osmocom_OBS_nightly and
never getting deleted). This script is now running automatically every
day.

The script does the following:
* Iterate over all OBS projects and packages inside the projects.
* Delete all source files matching *.dsc, *.tar.xz in old revisions from
  disk, which are not in the most recent revision anymore.
* Write a marker file with the revisions already iterated through (per
  project+package) and skip those on the next run.

Related: SYS#7407
Change-Id: I4649dedf0bb52f0bbdc7577ac0bc9e93cb3be192