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.

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.

Master will be simulated in test, and it will be used to give tasks to
slapgrid and read its responses.

Override some methods and modules in order to get list of what would be done,
instead of having it done.

Thanks to this blackbox testing is much easier.

It makes blackbox testing much easier, as own OS implementation can overridden
in tested class.