Fix the support of RestClient Node Sniffer for OpenSearch 2.x, and update unit tests for OpenSearch.
The current code contains the logic to be compatible with Elasticsearch 2.x version, which is conflict with OpenSearch 2.x, so removed that part of legacy code.
* Update the script create_test_nodes_info.bash to dump the response of Nodes Info API GET _nodes/http for OpenSearch 1.0 and 2.0 version, which used for unit test.
* Remove the support of Elasticsearch version 2.x for the Sniffer
* Update unit test to validate the Sniffer compatible with OpenSearch 1.x and 2.x
* Update the API response parser to meet the array notation (in ES 6.1 and above) for the node attributes setting. It will result the value of `node.attr` setting will not be parsed as array in the Sniffer, when using the Sniffer on cluster in Elasticsearch 6.0 and above.
* Replace "master" node role with "cluster_manager" in unit test
Signed-off-by: Tianli Feng <ftianli@amazon.com>
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