Set munin-async restart time to 10sec

Sometimes munin-async fails to start, presumably because it cannot
connect to the running munind yet.  The service file tells it to
restart always, but with the default sleep time before a restart of
100ms we often run into
 systemd[1]: munin-async.service: Start request repeated too quickly.
after 5 fails attempts within a second or two.

Give munind more time to actually launch.