Added possibility to create custom scripts

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']
}