Adjust attempt and wait durations

* Sleep for less time, check more often.

* Assume that 'ok on first attempt' means CDN already has origin marked
  as ok.

* Also adjust loop logic so that we aren't checking maxRetries twice.
Use 'GET /?cacheBuster=%s' for origin check req

This allows us to run the tests against a live environment, as
a 200 will be returned, but the origin will still need to be checked.
Merge pull request #9 from alphagov/origin_check_improve

Improve confirmOriginIsEnabled