This argument must work just as instance root parameter.
Well, it can be provided by command line and by slapos configuration file.
 - command line: signature-private-key-file=/path/to/the/file
 - slapos configuration file: signature_private_key_file=/path/to/the/file

Additionaly fix small issue with test assertions.

Mock netifaces in order to simulate real behaviour of machine. Add another
part of "persistence", which allows simulation of network stack.

Make test more flexible and controlallbe by exposing groups.

Additionally add required "persistence" to test.

Duplicate Listbox_asHTML from xhtml skin in vifib_hosting_theme, change checkbox buttons to radio buttons.

Possible this test detected another failing use case during destruction.
That's why it is still marked with NotImplementedError

Well the argument signature_private_key_file is supported as:
  - comand line: --signature-private-key-file
  - configuration file: signature_private_key_file

It is already requirement for slapos.core, and it is well known code.

Ue multiprocessing instead of threading, as there is no other way to kill
Flask application from code except SIGTERMing it.

Thanks to SOSS it will be possible to feed slapgrid* with various tasks, which
will allow to test different scenarios.