Role based EC2 access to S3

## Feature Request

The key parameter is required to access S3 using S3StorageDriver https://github.com/apache/libcloud/blob/7b3f55a3ac2ec8423555cd9bac3a42697ffc502c/libcloud/storage/drivers/s3.py#L1224, even if provided it fails down in the code in the absence of secret here https://github.com/apache/libcloud/blob/trunk/libcloud/common/aws.py#L313

Can you suggest the usage of your client for role-based EC2 access to S3 or the possibility to add it? (where key and secret are now known well in advance but using STS for temporary access by tokens)

Thanks!

Start with the S3StorageDriver code in libcloud/storage/drivers/s3.py at the referenced key requirement, then trace credential handling in libcloud/common/aws.py around the referenced line. Determine how role-based EC2 access and temporary STS tokens should be supported or documented; done means the intended usage or implementation scope is clear.

Written by the indexing model from the issue text.