[Feature Request] environment_name variable option for log_group_name in Logs section

For our use case, we have the cloudwatch agent sending the logs from EC2 instance managed by AWS Beanstalk.

Apart from the pre-configured standard logs provided by the Beanstalk environment, we want to send other logs as well. We have multiple environments (dev/qa/prod) that send logs to Cloudwatch Logs and the defaults do provide the environment name in the `log_group_name` which is helpful.

I have tried to replicate something similar for my custom logs before realizing that we can only provide `{instance_id}`, `{hostname}`, `{local_hostname}`, and `{ip_address}` as variables within the name from the [docs](https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/CloudWatch-Agent-Configuration-File-Details.html#CloudWatch-Agent-Configuration-File-Logssection).

I did try something like
```json
{
"logs": {
"logs_collected": {
"files": {
"collect_list": [
{
"file_path": "/va/log/my_custom.log",
"log_group_name": "{ 'Fn::Sub': '/aws/elasticbeanstalk/${AWSEBEnvironmentName}/var/log/my_custom.log' }",
"log_stream_name": "{instance_id}"
},
...,
]
}
}
}
}

```
and a few other combinations but it was not fruitful.

It would be very helpful if something like the `{environment_name}` can also be added as an option which makes it a lot easier to distinguish the different beanstalk environment logs that have the same name as opposed to using the `instance_id` or something else.

The issue names no source file or test. Start by tracing the Logs configuration handling and the linked CloudWatch Agent logs configuration documentation; done means a supported environment-name option is documented and covered by tests for custom log group names.

Written by the indexing model from the issue text.