AWS Batch: use custom log group name when requesting task logs, if it is defined

[nathanthorpe]

Nextflow allows the aws.batch.logsGroup parameter to be defined to override the log group name. The getTaskLogStream method is only able to use the default log group /aws/batch/job.

Update this method to allow support for retrieving logs when the custom log group name is defined.

Also update the value of the default log group name.