Halve the number of munin-html jobs, to prevent OOM errors due to fork count.

Should prevent mails like these:

Cron <munin@menotti> nice /usr/share/munin/munin-html
Killed

Cron <munin@menotti> nice /usr/share/munin/munin-html
fork failed: Cannot allocate memory at /usr/share/perl5/Munin/Master/HTMLOld.pm line 776.