Also merge data_cleanup tasks with syspreps and did a major cleanup

All printing had to be reordered and now it is done from a central point.

This commit fixes a bug where size method in DiskDevice returned the
payload size of the device one byte shorter than it actually was.

All modules should use image_creator.FatalError exception to report
fatal errors.

In the constructor only variable initialization is performed. This is
needed because the constructor is executed before the DiskDevice
instance is appended to the device list, and if something fails there,
then the resources may be cleaned up in an incorrect order.

The package is buggy and we can use pbs to do the same thing.

This is needed to suppress the StopIteration exception

This fixes cases where a normal user tries to execute the program and
the program raises pbs.CommandNotFound exceptions because it tries to
import a command from sbin, which is not in the path.

Hooray! This is the first release that actually works!

* 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

Make the cow file sparse
Add method in Device to list the partitions
Add a "is bootable" flag in Device constructor