Use the behavior established in the botocore python library to inform
how and when instance metadata is fetched in an attempt to acquire valid
AWS credentials.
- Use it as a fallback when no other credentials are provided
- Set the timeout of metadata requests to 1 second
Add option (AWS_USE_INSTANCE_ROLE) to have the AWS DNS API driver pull
the necessary credentials from the AWS EC2 instance metadata endpoint
when required.
This is a non-breaking change as it only takes effect when explicitly
turned on via the environment variable, and fails safe back to the
normal code path.