zabbixservers -> zabbixserver

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.

zabbix-agent/templates/zabbix_agentd.conf.j2

@@ -92,7 +92,7 @@ EnableRemoteCommands=1
 # Default:
 # Server=
 
-Server={% for server in zabbixservers %}{{ server }}, {% endfor %}
+Server={{zabbixserver}}
 
 ### Option: ListenPort
 #	Agent will listen on this port for connections from the server.
@@ -133,7 +133,7 @@ Server={% for server in zabbixservers %}{{ server }}, {% endfor %}
 # Default:
 # ServerActive=
 
-ServerActive={% for server in zabbixservers %}{{ server }}, {% endfor %}
+ServerActive={{zabbixserver}}
 
 ### Option: Hostname
 #	Unique, case sensitive hostname.