T5448: Add service zabbix-agent version 2

Add service zabbix-agent

set service zabbix-agent directory '/config/zabbix/'
set service zabbix-agent limits buffer-flush-interval '8'
set service zabbix-agent limits buffer-size '120'
set service zabbix-agent log debug-level 'warning'
set service zabbix-agent log size '1'
set service zabbix-agent server '192.0.2.5'
set service zabbix-agent server-active 192.0.2.5 port '10051'
set service zabbix-agent server-active 2001:db8::123