change cronjob for counting rows of database to be on the 2nd day of month, to avoid a race condition

LogType=console fails, since it requires the process to run in the
foreground. For now, log to a file again.

With buster the path to zabbix agent config files changed from
/etc/zabbix/zabbix_agentd.d/ to /etc/zabbix/zabbix_agent.conf.d/.
Also, the log directory changed from /var/log/zabbix/ to
/var/log/zabbix_agent, but after this change the agent logs to stdout
(journald) instead.

Although the config formats allows for it, we do not want to have more
than one zabbix server. Not having a list containing just one element
simplifies the configuration.

This is deprecated for 2.7