Adding support for :upsert for single writes, as well as document_id option

[gauravaradhye]

:upsert boolean value will upsert the document in case of single writes (not bulk) and document_id can be passed as custom object id from logstash so that it can be treated as custom "_id" in case of new Object ID.

Change-Id: Ic6899e5bb8537f85c7f9231d7e55576fd4d4777b

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