[Utils] Add missing import statement.
[Setup] Fix variable name.

zabbix_conf ->
zabbix_conf_httpd
[Assets] Add config file of Zabbix server.
[Setup] Add a new function to install configuration files of Zabbix
server: install_config_files_of_zabbix
[Setup] Use for statement in install_config_files_of_zabbix() function.