Pass region into credentials provider

[munkyboy]

This passes the s3.region configuration value into credential providers. It also updates the AwsAssumeRoleCredentialsProvider to specify a region when building a AWSSecurityTokenServiceClient.

Fixes #366

[cla-assistant]

All committers have signed the CLA.

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}