Feat/initial autotesting

[Cahb]

commit Testing: introduce initial automated testing core changes

Implement initial core for automated tests:
 * mostly resuses/extends existing simulator, kafka producer etc;
 * helper scripts added for manual testing
   (single infra/group add/del etc)
 * Tests compose either of simple steps premade into fixtures,
   or combine usage of both fixtures + custom code inside
   test's funcion

commit Client simulator: use pool of macs with single crt/key

Make client simulator use single cert/key pair with mac
being taken from predefind macs list. This makes it easier
to use simulator, as it doesn't depend upon a big list
of certificates that are expected to match simulated MAC
and so on.

Fixup some python imports to not use relative import,
but rather istruct makefile to pass python path var
that includes relative path.
This is a change required for following commits that
bring automated testing using this simulator.

[Cahb]

FYI: todo:
More complex tests, like make sure that not only expected messages / results appear, but there are no intermediate unexpected messages received from CGW;
Also some more sophisticated scenarios, like command handling (implement core-part of command handling) - simulate reboot, simulate cfg apply fail etc;