Trimmed the `elasticsearch.yml` configuration file

The main `elasticsearch.yml` file mixed configuration, documentation
and advice together.

Due to a much improved documentation at <http://www.elasticsearch.org/guide>,
the content has been trimmed, and only the essential settings have
been left, to prevent the urge to excessive over-configuration.

Related: 8d0f1a7d123f579fc772e82ef6b9aae08f6d13fd