Add `AWS_REGION` env for S3 in TensorFlow

This fix tries to address the issue raised in 14951 where
the region can only be specified with non-common `S3_REGION`
environment variables.

This fix adds the support of `AWS_REGION` which takes precedence
over `S3_REGION`.

This fix fixes 14951.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>