- Nest the configuration class in HealthIndicatorAutoConfiguration,
   bringing it into line with the other health indicator configuration
   classes
 - Include the statistics from the response in the health’s details
 - Map YELLOW to UP rather than UNKNOWN as it indicates that the cluster
   is running but that “the primary shard is allocated but replicas are
   not” [1]. The details can be used to determine the precise state of
   the cluster.
 - Add a property to configure the time that the health indicator will
   wait to receive a response from the cluster
 - Document the configuration properties
 - Update the tests to cover the updated functionality

See gh-2399

[1] http://www.elastic.co/guide/en/elasticsearch/reference/1.x/cluster-health.html