Detect DynamoDB region from EC2 instance profile

[ghost]

Is your feature request related to a problem? Please describe.
We deploy Vault to two different regions in EC2 and use the DynamoDB backend in both regions. Each Vault instance has its own table that is in the same region. The tables are configured the same in each region and they both have the same name. As a result, the Vault configuration file is the same in both regions except for the region property required by the DynamoDB storage configuration. Maintaining two separate config files, one for each region, that only differ by one line seems unnecessary when the region information can be obtained dynamically.

Describe the solution you'd like
It would be great if Vault would use the EC2 instance metadata could be used to dynamically determine the default AWS region when one isn't explicitly specified. It looks like the same functionality was added to the AWS KMS seal back in January (Pull Request 6025). The changes to the Dynamo backend code are probably very similar.

Describe alternatives you've considered
We have a script that runs prior to running Vault to ensure the correct, region-specific configuration file will be loaded by Vault.

Explain any additional use-cases
N/A

Additional context
N/A