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Also merge data_cleanup tasks with syspreps and did a major cleanup

This can be used to create a set of by default disabled data_cleanup
and sysprep tasks.

* Add foreach_file function in OSBase. This is used by all cleanup
  methods
* Add slackware class. This class derives from linux and overwrites the
  log cleanup behaviour. This is needed since slackware stores all
  package management info in /var/log/packages
* Add a shrink method in DiskDevice. This method shrinks the last
  filesystem to the minimum size
* Make the code style comply with pep8