[event log] use require_alias when indexing event log documents

[pmuellr]

In the past, when our event log bootstrapping code was not robust enough, we would occasionally have Kibana start with the event log alias not set up, and then subsequent indexing of events would end up writing the documents to an index with the name of the alias. Very problematic.

We've since made the bootstrapping code more robust, but as an additional safety check, we could use the require_alias option to make sure we never create an index with the name of the alias. I assume there is some way of using this with bulk requests ...

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)