Add support for us-east-2 AWS region

[dadoonet]

AWS supports us-east-2 region. We need to add it to our s3 and ec2 plugins.

• EC2 region: http://docs.aws.amazon.com/general/latest/gr/rande.html#ec2_region
• S3 region: http://docs.aws.amazon.com/general/latest/gr/rande.html#s3_region

Note that in the meantime people can define cloud.aws.ec2.endpoint and cloud.aws.s3.endpoint to ec2.us-east-2.amazonaws.com and s3.us-east-2.amazonaws.com.

[dadoonet]

Closed by #21961