fix for [#20]

[samuk10]

Same as: #20
On ubuntu i have no problem on the installation, but on centos7 i had this problem.
running the scripts was everythin fine but using agent this happen:

$ /etc/zabbix# zabbix_agentd -t "systemd.service.status[ssh]"
systemd.service.status[ssh] [m|ZBX_NOTSUPPORTED] [Unsupported item key.]

Fix:

1. Find in agend.conf which folder for external confs

$ cat /etc/zabbix/zabbix_agentd.conf | grep /etc/zabbix/zabbix_agent

2. Place userparameter_systemd_services.conf inside the folders above
3. Restart zabbix_agent

For me was like this:
Zabbix Agent2 on Ubuntu Working in this folder: /etc/zabbix/zabbix_agent2.d/userparameter_systemd_services.conf
Zabbix Agent(old): on Centos7 Working in this folder: /etc/zabbix/zabbix_agentd.conf.d/userparameter_systemd_services.conf