[CHORE] Investigate if --region should drive region_name in boto3 Session creation

[malachi-constant]

Issue

If using a partition outside of aws & AWS_DEFAULT_REGION is not set, seedfarmer will fail on Session creation as sts.get_caller_identity() will fail.

Decision point

• Should --region in seedfarmer apply {manifest_path} --region cn-north-1 be passed to get_caller_identity()?

Potential Implementation

sts.get_caller_identity(region_name=region) if partition !="aws" else sts.get_caller_identity()

[malachi-constant]

Will close this unless it's deemed necessary.