This is to provide a clean path to future extension work such as adding
a _use_container_role function to offer similar support for ECS
containers.
The $_using_role flag has also been made generic so that future role
providers can also make use of it.
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.
