Setup Elasticsearch cluster

Add the following configuration to `config/elasticsearch.yml` in each instance of Elasticsearch:

cluster:
  name: some-log

network:
  host:
    - _eth1_
    - _local_

discovery.zen.ping.unicast.hosts: ["1.2.3.4", "1.2.3.5", "1.2.3.6", "1.2.3.7", "1.2.3.8"]
discovery.zen.minimum_master_nodes: 1

Note the value of `discovery.zen.minimum_master_nodes` is used for simplicity. Based on the recommendation it will be 3:

# Prevent the "split brain" by configuring the majority of nodes (total number of nodes / 2 + 1):
#
# discovery.zen.minimum_master_nodes: 3