Revert "Only set discovery seed hosts if greater than 1"

[pquentin]

Reverts #77

As soon as this was merged, this broke our nightly benchmarks with the following error message:

[2023-01-18T11:56:56,877][INFO ][o.e.t.TransportService ] [rally-node-0] publish_address {192.168.20.29:9300}, bound_addresses {192.168.20.29:9300}
[2023-01-18T11:56:56,985][INFO ][o.e.b.BootstrapChecks ] [rally-node-0] bound or publishing to a non-loopback address, enforcing bootstrap checks
bootstrap check failure [1] of [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

Indeed, the way Elasticsearch differentiates development vs. production mode is the address that Elasticsearch is bound to, not the number of nodes in the cluster: https://www.elastic.co/guide/en/elasticsearch/reference/8.6/bootstrap-checks.html#dev-vs-prod-mode

cc @inqueue @danielmitterdorfer for awareness