How to implement SIGHUP tests

[Tomek Mrugalski]

Here's a summary of our (Marcin, Stephen, Tom, Tomek) discussion on Jabber:

   We'll remove python tests that start kea4,6, D2 with various
   parameters.

   We'll keep signal tests written in shell.

   If we decide later that we should add more system-level tests to
   Kea repo, they should be written in shell.

I'll update #3406 ticket with that info (in essence revoke my suggestion
to rewrite signal tests).

Tomek