Add killasgroup=true to the supervisord configuration file example for celeryd

[staaas]

When we stop celeryd with KILL signal, children processes stay alive.
The problem was discussed 2 years ago here: issues/102

supervisord now has a special key killasgroup in order to handle this situation:

If true, when resorting to send SIGKILL to the program to terminate it send it to its whole process group instead, taking care of its children as well, useful e.g with Python programs using multiprocessing.
Introduced: 3.0a11

This key is ignored for older supervisord versions.

Just added killasgroup to the celeryd configuration example

[ask]

Thanks! Merged!