scion.sh: block for child processes at shutdown (#4445)

The `supervisorctl shutdown` command does not block for the supervisor
or its child processes to terminate.
This can occasionally lead to situations where SCION processes were
still running after the `scion.sh stop` command returned. In the CI
system, which immediately proceeds to bundle up the log files, this led
to `tar` reporting an error that the log file was still being written
to.
Fixed by invoking `supervisorctl stop all`, which does block, to
terminate all child processes before `shutdown`.

scion.sh

@@ -113,7 +113,8 @@ stop_scion() {
     if is_docker_be; then
         ./tools/quiet ./tools/dc down
     else
-        ./tools/quiet tools/supervisor.sh shutdown
+        ./tools/quiet tools/supervisor.sh stop all # blocks until child processes are stopped
+        ./tools/quiet tools/supervisor.sh shutdown # shutdown does not block, but as children are already stopped, actual shutdown will be prompt too.
         run_teardown
     fi
 }