Provide a mechanism to deal with the rate limit in DigitalOcean

The DigitalOcean v2 API is rate limited. The limits are not particularly large, so jclouds should provide a mechanism to automatically deal with them. See: https://developers.digitalocean.com/documentation/v2/#rate-limit

Ideally this should be an optional component that users can choose to enable. When enabled, requests that failed due to the rate limit should be enqueued and retried when the rate limit resets, and when disabled, requests should fail and a meaningful exception should be propagated.