Description and code how to deploy and integrate External Zabbix server

[tyloch]

We should show how to deploy "external" Zabbix server and integrate it with this environment.

[thomnico]

WIP

[tyloch]

any progress as current description is unclear.
e.g.:

1. which Zabbix charm should be used to deploy external Zabbix server. We have 2 Zabbix-servers in charm store.
2. Security group - I assume we need to open it only for Zabbix agent. There is no need to create new as on AWS there we have security groups.