Support `CreateLogStream` without `CreateLogGroup`

[skeggse]

Problem

I'd rather only provide access to some machines to create log streams within an existing log group, rather than also providing access to CreateLogGroup; would it be reasonable to have a separate auto_create_group flag that we can set to false to disable the group creation portion of that functionality?

[cristian-radu]

I am also in favor of this. I would like to manage the log groups through Terraform IaC and just let this plugin create the log streams one per unique EC2 instance-id for example. Was not expecting the log groups to automatically show up and had to do some cleanup due to this.

https://github.com/fluent-plugins-nursery/fluent-plugin-cloudwatch-logs/pull/254