[Feature]: Make `elasticsearch_network_host` more flexible

Describe the feature request

In #315 we stumbled over a potential suite of problems:

• elasticsearch_network_host now is a string that may or may not contain an array of network addresses and macros. (e.g. 192.168.15.5 or `"local", "site", "elastic01".
• In our current code, the string is used "as is" and contain what ever Elasticsearch can take in
• In #315 the contents of the variable is to be used to connect to Elasticsearch via curl or uri module. This means it could only hold a network address
• If we make a list instead of a string, we could easily choose one of the addresses for connection. But we would break compatibility to existing setups. (Bad but doable, since we're not on 1.0.0, yet)
• If we make it a list, we need to find a way to choose one that's actually reachable via network and not a macro

The only solutions I can think of for now is to introduce another variable that holds a list. Then parse the list, find a network address and use this as a default for elasticstack_network_host. And replace the appearance of the variable in templates with the new one. Or leave anything as it is and introduce a new variable like elasticsearch_network_host_connect when trying to reach the host.

Start with the compatibility concerns and alternatives described in issue #317, then read issue #315 for the connection requirements. Trace every use of elasticsearch_network_host, including its use with curl or the uri module and in templates. Done means a decided, documented approach that preserves or intentionally replaces existing behavior and selects a usable connection address.