Add initial_master_nodes

[in0rdr]

Elasticsearch 7.x comes with new parameters for discovery of cluster nodes: https://www.elastic.co/guide/en/elasticsearch/reference/current/discovery-settings.html

If network.host: 0.0.0.0 is set, Elasticsearch will not start without setting initial_master_nodes as well:

#/etc/elasticsearch/elasticsearch.yml
cluster.initial_master_nodes:
  - "elastic0"

If this is not the case, Elasticsearch will fail to start with the following message:

[2019-07-03T14:23:13,130][ERROR][o.e.b.Bootstrap          ] [elastic0] node validation exception
[1] bootstrap checks failed
[1]: the default discovery settings are unsuitable for production use; at least one of [discovery.seed_hosts, discovery.seed_providers, cluster.initial_master_nodes] must be configured

I did not check on the other parameters, seed_hosts or seed_providers. Probably they could be implemented in the same "task". However, a single-node Elasticsearch bootstraps fine by defining initial_master_nodes only.

[in0rdr]

This could be resolved with https://github.com/geerlingguy/ansible-role-elasticsearch/issues/38 as well, because it concerns "append only" arguments for newer versions of Elasticsearch

[dale-c-anderson]

@in0rdr , agreed - https://github.com/geerlingguy/ansible-role-elasticsearch/issues/38 was a great suggestion. Simple and effective.

Pull request has been submitted https://github.com/geerlingguy/ansible-role-elasticsearch/pull/54

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