Fix role assumer region issue

[petervandoros]

Context

There is an error when assuming a role without having configured a default AWS region in your environment:

Executing apply on my-stack in us-east-1
Aws::Errors::MissingRegionError: missing region; use :region option or export region name to ENV['AWS_REGION']

Changes

• Use the CloudFormation driver's region, which includes fallbacks when it's not specified, when assuming a role

Considerations

Which region is used doesn't actually matter when assuming a role, as IAM and STS are global services.

[orien]

It seems the specs are suffering from a similar problem to that described in the PR description: when a default region is defined in ~/.aws/config the tests pass (a string value is passed to the Aws::AssumeRoleCredentials initialiser). But if no default region is defined, the tests fail (nil is passed to the Aws::AssumeRoleCredentials) as exhibited in CI.

Perhaps StackMaster.cloud_formation_driver should be stubbed.

[orien]

🎉