webserver: try using zabbix vars only when using zabbix integration

webserver/defaults/main.yml

@@ -7,4 +7,6 @@ webserver_enable_ipv6: true
 webserver_resolver: "{{ nameservers }}"
 disallow_apache2: true
 webservers: []
-zabbix_password: "{{ lookup('passwordstore', zabbix_user) }}"
+
+# If you use the Zabbix integration, define this variable.
+# zabbix_password: "{{ lookup('passwordstore', zabbix_user) }}"