Allow auto-detection of AWS region when using the vault CLI

[kalafut]

This PR lets the CLI detect the AWS region based on standard AWS credentials precedence (e.g. environment variables, or ~/.aws_credentials) if the region is specified as auto. Previously you either had to provide the region manually, or accept the default of us-east-1.

The region calculation was verified manually (using by temporarily logging the computed region):

➜  vault git:(aws-region-detect) ✗ vault login -method=aws
us-east-1

➜  vault git:(aws-region-detect) ✗ vault login -method=aws region=
us-east-1

➜  vault git:(aws-region-detect) ✗ vault login -method=aws region=abc123
abc123

➜  vault git:(aws-region-detect) ✗ vault login -method=aws region=auto
us-west-2