Add overrides to health check class
Reduce chance of sending unstable Fastly stats to cloudwatch

Move the query window of the Fastly api query to avoid incomplete blocks.
Set the default timestamps to reflect the new query window.
Reduce the fastly job frequency, and give a simple text reponse in the controller.
Merge pull request #1231 from guardian/fastly-stats

Get more accurate data from Fastly stats api