Add Elasticsearch Cluster Health Check

[stefannikolei]

This is a second test for elasticsearch. The one implemented now just pings elastic. This one really checks the cluster health. It only respond with unhealthy when the cluster health endpoint answers with red.

Here is the documentation from elastic to this topic:

https://www.elastic.co/guide/en/elasticsearch/reference/current/cluster-health.html

[sungam3r]

Thanks for contribution. I suggest to implement this feature in the existing healthcheck by providing new property in ElasticsearchOptions like bool UseClusterHealthAPI { get; set; } - less code.