Taking AWS_REGION env as default region

[sivagollapalli]

FIX #2112 issue
By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

1. To make sure we include your contribution in the release notes, please make sure to add description entry for your changes in the "unreleased changes" section of the CHANGELOG.md file (at corresponding gem). For the description entry, please make sure it lives in one line and starts with Feature or Issue in the correct format.

2. For generated code changes, please checkout below instructions first:
   https://github.com/aws/aws-sdk-ruby/blob/master/CONTRIBUTING.md

Thank you for your contribution!

[alextwoods]

The SDK will already look for region (and other config values) in the ENV before other sources (See this dev guide for an overview of the order of preference or to see the code itself: CredentialProviderChain).

[mullermp]

This doesn't fix the issue unfortunately. :( Thanks for opening the PR.

[sivagollapalli]

@mullermp No problem. I will try to fix.

[alextwoods]

@sivagollapalli - I don't think we should be changing the default behavior of Aws.config here (without a major version bump) as this could break current users. Aws.config is easy to update in application code, so users are not blocked from doing this themselves if its behavior they want.

[sivagollapalli]

@alextwoods Would that mean are we discarding this issue #2112 or we are postponing till next major release?