Ignore SIGINT/TERM at kas level, let sub-processes handle it

This seems to be the cleanest and simplest way of properly terminating
kas while one or more sub-processes are running: just ignore the signal.
The sub-processes will get it as well, and they will terminate
themselves properly. kas will get an error then and shut down. Problem
solved.

Forwarding the signal didn't work reliably anyway because we couldn't
count on kas being a process group leader in all cases.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas/kas.py

@@ -83,18 +83,15 @@ def create_logger():
 
 def interruption():
     """
-        Handle SIGINT/SIGTERM
+        Ignore SIGINT/SIGTERM in kas, let them be handled by our sub-processes
     """
-    sys.exit(1)
+    pass
 
 
 def _atexit_handler():
     """
-        Close event loop and terminate the whole process group
+        Wait for completion of the event loop
     """
-    signal.signal(signal.SIGTERM, signal.SIG_IGN)
-    os.killpg(os.getpid(), signal.SIGTERM)
-
     loop = asyncio.get_event_loop()
     pending = asyncio.Task.all_tasks()
     loop.run_until_complete(asyncio.gather(*pending))