AwsAssumeRoleCredentialsProvider fails with region missing

[AyoubOukh]

We've set up io.confluent.connect.s3.auth.AwsAssumeRoleCredentialsProvider like so

s3.credentials.provider.class=io.confluent.connect.s3.auth.AwsAssumeRoleCredentialsProvider
s3.credentials.provider.sts.role.arn=arn:aws:iam::xxxxxxxx:role/my-role
s3.credentials.provider.sts.role.session.name=session-name
s3.credentials.provider.sts.role.external.id=external-id

and get the following error

[Worker-0f3f4a9ad02a8f900] Caused by: com.amazonaws.SdkClientException: Unable to find a region via the region provider chain. Must provide an explicit region in the builder or setup environment to supply a region.

This was also spotted here btw: #366