Check if the signature_private_key_file is well handled during the argument parsing.

You can use the slapgrid scripts without providing the signature_private_key_file.

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