* Use us-gov-west-1 for global APIs in aws-us-gov

Certain partition-global AWS services, like IAM, seem to require
specific regions. In the regular 'aws' partition, this is us-east-1. In
the 'aws-us-gov' partition, this is us-gov-west-1. Providing
us-gov-east-1 returns an error from AWS:

  SignatureDoesNotMatch: Credential should be scoped to a valid region, not 'us-gov-east-1'.

This resolves a problem where AWS authentication could randomly fail
depending on the value cached by Vault at startup.