[core] Fixes #434

+ Fix inspired from
http://stackoverflow.com/questions/4789837/how-to-terminate-a-python-subprocess-launched-with-shell-true/4791612#4791612.
+ Kills the process group leader, whose signal is transmitted to all of the child processes of the group

framework/shell/blocking_shell.py

@@ -128,7 +128,7 @@ def shell_exec_monitor(self, Command, PluginInfo):
             logging.warn(outdata)
             Output += outdata
             try:
-                os.kill(proc.pid, signal.SIGTERM)  # Plugin KIA (Killed in Action)
+                os.killpg(os.getpgid(proc.pid), signal.SIGTERM)  # Plugin KIA (Killed in Action)
             except OSError:
                 pass  # Plugin RIP (Rested In Peace)
             Cancelled = True