This prevents find from failing if other parts of the instance have
wrong permissions.

/reviewed-on !51

source and exclude can be in artbitrary order, so sort them in order to have
stable calls.

This simplifies assertions in tests and is no-op for the code.

It is important for DevOps to understand which files were modified during
synchronisation procedure in order to track issues fast.

and not in the backuped files.
If we look in the backup files, then of course we find modified files, which are
the ones that have just been copied from the instance folders

To provide uselful functions that can be reused in other scripts
used in the context of webrunner's resiliency

Rewrite the [runner-exporter bash script](https://lab.nexedi.com/nexedi/slapos/blob/2a01586bfe17f46e2ef26ec6e45bef692435ed28/software/slaprunner/template/runner-export.sh.jinja2) in python.

Also parametarize the signature function to not use only sha256sum which can create issues on some systems. This should be a separated commit, but that shouldn't be a blocker.

Write unit tests for the main functions of this exporter script. As every trick in the original bash script were not obvious / understood / found by me, I may have missed some.

/reviewed-on !44