Adds properties that affect Elasticsearch index scalability

[codefromthecrypt]

This adds the following properties, which help control the scalability
of Zipkin's index in Elasticsearch.

* `ES_INDEX_SHARDS`: The number of shards to split the index into. Each shard and its replicas
                     are assigned to a machine in the cluster. Increasing the number of shards
                     and machines in the cluster will improve read and write performance. Number
                     of shards cannot be changed for existing indices, but new daily indices
                     will pick up changes to the setting. Defaults to 5.
* `ES_INDEX_REPLICAS`: The number of replica copies of each shard in the index. Each shard and
                       its replicas are assigned to a machine in the cluster. Increasing the
                       number of replicas and machines in the cluster will improve read
                       performance, but not write performance. Number of replicas can be changed
                       for existing indices. Defaults to 1. It is highly discouraged to set this
                       to 0 as it would mean a machine failure results in data loss.

[codefromthecrypt]

This came to mind when troubleshooting #1141

@anuraaga PTAL, particularly on the descriptions.

[anuraaga]

While in some sense it "limits", I think it's a bit too semantic.

The number of shards to split the index into. Each shard and its replicas are assigned to a machine in the cluster. Increasing the number of shards and machines in the cluster will improve read and write performance. Number of shards cannot be changed for existing indices, but new daily indices will pick up changes to the setting. Defaults to 5.

[codefromthecrypt]

updated with better comments thx to @anuraaga