Symbiosis: Logrotate cron error for prosody when it's not running

Imported from https://www.github.com/BytemarkHosting/symbiosis/issues/131

The logrotate cron will email the following warning every week if prosody isn't active:

/etc/cron.daily/logrotate:
error: error running shared postrotate script for
'/var/log/prosody/prosody.log /var/log/prosody/prosody.err '
run-parts: /etc/cron.daily/logrotate exited with return code 1

It looks like this is because the postrotate tries to check for the existence of /var/run/prosody/prosody.pid which won't be there when prosody is disabled (by default):

[ -e /var/run/prosody/prosody.pid ] && /etc/init.d/prosody reload > /dev/null

We should be able to suppress that by changing this line to e.g

/etc/init.d/prosody reload > /dev/null