S3 Input/Output with temporary credentials (IAM Roles)

[Vladimir Stoyak]

Our setup involves AWS IAM roles when with permanent access_key and 
access_secret we need to assume specific role (ie getting temporary credentials 
to use AWS resources).
I was wondering what would be the best way handling this, ie how to set 
fs.s3n.awsAccessKeyId and fs.s3n.awsSecretAccessKey programmatically and also 
how to handle expired sessions.
Thanks,Vladimir