use shred path as parameter and print stderr message when shred command fail

/reviewed-on nexedi/slapos.toolbox!18

/reviewed-on !16

/reviewed-on nexedi/slapos.toolbox!15

/reviewed-on nexedi/slapos.toolbox!14

Documentation on it should be provided on slapos.libnetworkcache.

  File can be empty for whatever reason, so handle it in the same way the file don't exist.

Prevent the method to loop indefinitely

/reviewed-on nexedi/slapos.toolbox!12

  In distribution systems, it is possible that master didn't indexed the Software Installation
  when we check, this change will consider we don't know the state of the Software Installation on
  this stage. The code will retry later anyway.

  Update the tests after the fix on the code, as the map now contains all the tests form all the groups

 This bug was preventing to get a single test per group (computer) 
 per time. Causing multiple tests run on the same computer and no
 tests run on others. 

  Implement a watchdog to kill process which are hold on dead lock
  state after a greceful reload.

qmp drive-backup command should exit when qemu has finished to backup image
else image is copied when qemu is still writing and result to a broken backup

If qemu return error in command, raise instead of silent exit with code 0

  This Workarround should be removed on future versions of webrunners

  In case haproxy were not working before.

  Export will fail if backup is run while export, so be more flexible.

  Missing import