Fix region handling for the provider

[outcatcher]

Summary of the Pull Request

• Use region values from provider config and schema
• Use OS_REGION env var as region default in provider schema
• Tests: make OS_REGION_NAME to use provider config

PR Checklist

• Refers to: [OBS] nil pointer dereference on bucket creation #690. Just related to, not actual fix.
• Tests added/passed.
• Documentation updated.

Acceptance Steps Performed

=== RUN   TestAccS3Bucket_basic
--- PASS: TestAccS3Bucket_basic (25.11s)
=== RUN   TestAccAWSS3MultiBucket_withTags
--- PASS: TestAccAWSS3MultiBucket_withTags (27.27s)
=== RUN   TestAccS3Bucket_namePrefix
--- PASS: TestAccS3Bucket_namePrefix (27.02s)
=== RUN   TestAccS3Bucket_generatedName
--- PASS: TestAccS3Bucket_generatedName (27.06s)
=== RUN   TestAccS3Bucket_region
--- PASS: TestAccS3Bucket_region (27.16s)
=== RUN   TestAccS3Bucket_Policy
--- PASS: TestAccS3Bucket_Policy (72.65s)
=== RUN   TestAccS3Bucket_UpdateAcl
--- PASS: TestAccS3Bucket_UpdateAcl (49.43s)
=== RUN   TestAccS3Bucket_Website_Simple
--- PASS: TestAccS3Bucket_Website_Simple (74.89s)
=== RUN   TestAccS3Bucket_WebsiteRedirect
--- PASS: TestAccS3Bucket_WebsiteRedirect (76.92s)
=== RUN   TestAccS3Bucket_WebsiteRoutingRules
--- PASS: TestAccS3Bucket_WebsiteRoutingRules (52.85s)
=== RUN   TestAccS3Bucket_shouldFailNotFound
--- PASS: TestAccS3Bucket_shouldFailNotFound (21.06s)
=== RUN   TestAccS3Bucket_Versioning
--- PASS: TestAccS3Bucket_Versioning (75.27s)
=== RUN   TestAccS3Bucket_Cors
--- PASS: TestAccS3Bucket_Cors (51.61s)
=== RUN   TestAccS3Bucket_Logging
--- PASS: TestAccS3Bucket_Logging (36.69s)
=== RUN   TestAccS3Bucket_Lifecycle
--- PASS: TestAccS3Bucket_Lifecycle (73.20s)
=== RUN   TestS3BucketName
--- PASS: TestS3BucketName (0.00s)
PASS

Process finished with exit code 0

[otc-zuul]

Build succeeded.

• golang-make-test : SUCCESS in 2m 35s
• golang-make-vet : SUCCESS in 2m 24s
• goreleaser-build : SUCCESS in 7m 04s