Added possibility to create custom scripts

[vladimir-low]

Such scripts could be used at user parameters later.

This feature is disabled by default. To start using it you need to do the following:

• enumerate all script you want to be uploaded in node['zabbix']['agent']['user_script'] list
• put scripts into files/default/scripts folder.

NOTE: filename should be the same as specified at user_script list

EXAMPLE: default['zabbix']['agent'] => { ... 'user_script' => ['check_zookeeper.py'], 'user_parameter' => ['zookeeper[*],/etc/zabbix/user_scripts/check_zookeeper.py -s localhost:2181'] }